THE ASPIRE CAPITAL TRANSACTION

General

On November 11, 2015, we entered into the Purchase Agreement which provides that, upon the terms and subject to the conditions and limitations set forth therein, Aspire Capital is committed to purchase up to an aggregate of $25.0 million of our shares of Common Stock over the term of the Purchase Agreement. Concurrently with entering into the Purchase Agreement, we also entered into the Registration Rights Agreement, in which we agreed to file one or more registration statements as permissible and necessary to register under the Securities Act, the sale of the shares of our Common Stock that have been and may be issued to Aspire Capital under the Purchase Agreement.

As of November 11, 2015, there were 30,446,260 shares of our Common Stock outstanding (25,667,728 shares held by non-affiliates), excluding the 6,086,207 shares offered that have not been issued but may become issuable to Aspire Capital pursuant to the Purchase Agreement. If all of the 6,086,207 shares of our Common Stock offered hereby were issued and outstanding as of the date thereof, such shares would represent 19.99% of the total Common Stock outstanding or 23.71% of the non-affiliate shares of Common Stock outstanding as of the date thereof. The number of shares of our Common Stock ultimately offered for sale by Aspire Capital is dependent upon the number of shares purchased by Aspire Capital under the Purchase Agreement.outstanding.

As of November 19, 2015, there were 30,446,260 shares of our Common Stock outstanding (25,667,728 shares heldDilution per share to new investors represents the difference between the amount per share paid by non-affiliates), excluding the 6,086,207 shares offered that have not been issued but may become issuable to Aspire Capital pursuant to the Purchase Agreement. The number of shares of our Common Stock ultimately offeredpurchasers for sale by Aspire Capital is dependent upon the number of shares purchased by Aspire Capital under the Purchase Agreement.

Pursuant to the Purchase Agreement and the Registration Rights Agreement, we are registering 6,086,207 shares of our Common Stock under the Securities Act, which we may issue to Aspire Capital after this registration statement is declared effective under the Securities Act. Under the Purchase Agreement, we have the right but not the obligation to issue more than the 6,086,207 shareseach share of Common Stock included in the Units in this prospectus to Aspire Capital. As ofoffering and the date hereof, we do not have any plans or intent to issue to Aspire Capital any shares of Common Stock in addition to the 6,086,207 shares of Common Stock offered hereby.

After the Securities and Exchange Commission has declared effective the registration statement of which this prospectus is a part, on any trading day on which the closing sale pricenet tangible book value per share of our Common Stock is not less than $0.10 per share, we haveimmediately following the right, in our sole discretion,completion of this offering.

After giving effect to present Aspire Capital with a Purchase Notice, directing Aspire Capital (as principal) to purchase up to 150,000 sharesthe sale of our Common Stock per business day, up to $25.0 million4,000,000 Units that are offered by this prospectus, excluding the proceeds if any from the exercise of our Common Stockthe Warrants included in the aggregateUnits, at a Purchase Price calculated by reference to the prevailing marketan offering price of our Common Stock over the preceding 12-business day period (as more specifically described below).

In addition, on any date on$1.20 per Unit, which we submit a Purchase Notice to Aspire Capital for 150,000 shares of our Common Stock and our stock price is not less than $0.50 per share, we also have the right, in our sole discretion, to present Aspire Capital with a VWAP Purchase Notice directing Aspire Capital to purchase an amount of stock equal to up to 30% of the aggregate shares of the Company’s common stock traded on the NASDAQ on the next trading day, subject to the VWAP Purchase Share Volume Maximum and the VWAP Minimum Price Threshold. The VWAP Purchase Price is calculated by reference to the prevailing market price of our Common Stock (as more specifically described below).

The Purchase Agreement provides that the Company and Aspire Capital shall not effect any sales under the Purchase Agreement on any purchase date where the closing sale price of our Common Stock is less than $0.10. There are no trading volume requirements or restrictions under the Purchase Agreement, and we will control the timing and amount of any sales of our Common Stock to Aspire Capital. Aspire Capital has no right to require any sales by us, but is obligated to make purchases from us as we direct in accordance with the Purchase Agreement. There are no limitations on use of proceeds, financial or business covenants, restrictions on future fundings, rights of first refusal, participation rights, penalties or liquidated damages in the Purchase Agreement. Aspire Capital may not assign its rights or obligations under the Purchase Agreement. The Purchase Agreement may be terminated by us at any time, at our discretion, without any penalty or cost to us. The Purchase Agreement provides that on the date of its execution, the certain Purchase Agreement, dated as of May 26, 2015, by and between the Company and Aspire Capital, was terminated.

Purchase of shares under the Purchase Agreement

Under the Purchase Agreement, on any trading day selected by us on which the closing sale price of our Common Stock exceeds $0.10 per share, we may direct Aspire Capital to purchase up to 150,000 shares of our Common Stock per trading day. The Purchase Price of such shares is equal to the lesser of:

In addition, on any date on which we submit a Purchase Notice to Aspire Capital for purchase of 150,000 shares and our stock price is not less than $0.50 per share, we also have the right to direct Aspire Capital to purchase an amount of stock equal to up to 30% of the aggregate shares of the our Common Stock traded on the NASDAQ on the next trading day, subject to the VWAP Purchase Share Volume Maximum and the VWAP Minimum Price Threshold, which is equal to the greater of (a) 80% of the closing price of our Common Stock on the business day immediately preceding the VWAP Purchase Date or (b) such higher price as set forth by the Company in the VWAP Purchase Notice. The VWAP Purchase Price of such shares is the lower of:

The purchase price will be adjusted for any reorganization, recapitalization, non-cash dividend, stock split, or other similar transaction occurring during the trading day(s) used to compute the purchase price. We may deliver multiple Purchase Notices and VWAP Purchase Notices to Aspire Capital from time to time during the term of the Purchase Agreement, so long as the most recent purchase has been completed.

Minimum Share Price

Under the Purchase Agreement, the Company and Aspire Capital may not effect any sales of shares of our Common Stock on any trading day that the closing sale price of our Common Stock is less than $0.10 per share.

Compliance with the NASDAQNasdaq Capital Market Rules

The Purchase Agreement provides that the number of shares that may be sold pursuant to the Purchase Agreement shall be limited to 6,086,207, or the Exchange Cap, which represents 19.99% of our outstanding shares as of November 11, 2015, unless shareholder approval or an exception pursuant to the rules of the NASDAQ Capital Market is obtained to issue more than 19.99%, to be in compliance with the applicable listing maintenance rules of the NASDAQ Capital Market. We are not required or permitted to issue any shares of Common Stock under the Purchase Agreement if such issuance would breach our obligations under the rules or regulations of the NASDAQ Capital Market.

Beneficial Ownership Limitation

Under the Purchase Agreement, we and Aspire Capital may not effect any sales of shares of our Common Stock if such shares proposed to be issued and sold, when aggregated with all other shares of our Common Stock beneficially owned by Aspire Capital and its affiliates, would result in the beneficial ownership by Aspire Capital and its affiliates of more than 19.99% of our then issued and outstanding shares of Common Stock.

Events of Default

Generally, Aspire Capital may terminate the Purchase Agreement upon the occurrence of any of the following events of default:

Minimum Share Price

Under the Purchase Agreement, we and Aspire Capital may not effect any sales of shares of our common stock under the Purchase Agreement on any trading day that the closing sale price of our common stock is less than $0.10 per share.

Our Termination Rights

The Purchase Agreement may be terminated by us at any time, at our discretion, without any cost to us.

No Short-Selling or Hedging by Aspire Capital

Aspire Capital has agreed that neither it nor any of its agents, representatives and affiliates shall engage in any direct or indirect short-selling or hedging, which establishes a net short position with respect to our Common Stock during any time prior to the termination of the Purchase Agreement.

Effect of Performance of the Purchase Agreement on Our Stockholders

The Purchase Agreement does not limit the ability of Aspire Capital to sell any or all of the 6,086,207 shares registered in this offering. It is anticipated that shares registered in this offering will be sold over a period of up to approximately 30 months fromand after deducting the date of this prospectus. The sale by Aspire Capital of a significant amount of shares registered in thisestimated underwriting discounts, commissions and offering at any given time could cause the market price ofexpenses, our Common Stock to decline and/or to be highly volatile. Aspire Capital may ultimately purchase all, some or none of the 6,086,207 shares of Common Stock not yet issued but registered in this offering. After it has acquired such shares, it may sell all, some or none of such shares. Therefore, sales to Aspire Capital by us pursuant to the Purchase Agreement also may result in substantial dilution to the interests of other holders of our Common Stock. However, we have the right to control the timing and amount of any sales of our shares to Aspire Capital and the Purchase Agreement may be terminated by us at any time at our discretion without any penalty or cost to us.

Amount of Potential Proceeds to be Received under the Purchase Agreement

In connection with entering into the Purchase Agreement, we authorized the sale to Aspire Capital of up to $25.0 million of our shares of Common Stock. However, we estimate that we will sell no more than 6,086,207 shares to Aspire Capital under the Purchase Agreement, all of which are included in this offering. Subject to any required approval by our Board of Directors, we have the right but not the obligation to issue more than the 6,086,207 shares included in this prospectus to Aspire Capital under the Purchase Agreement. In the event we elect to issue more than 6,086,207 shares under the Purchase Agreement, we will be required to file a new registration statement and have it declared effective by the SEC. The number of shares ultimately offered for sale by Aspire Capital in this offering is dependent upon the number of shares purchased by Aspire Capital under the Purchase Agreement. The following table sets forth the number and percentage of outstanding shares to be held by Aspire Capital after giving effect to the sale of shares of Common Stock issued to Aspire Capital at varying purchase prices:

DILUTION

The sale of our common stock to Aspire Capital pursuant to the Purchase Agreement will have a dilutive impact on our stockholders. As a result, our net income per share, if any, would decrease in future periods and the market price of our Common Stock could decline. In addition, the lower our stock price is at the time we exercise our right to sell shares to Aspire Capital, the more shares of our Common Stock we will have to issue to Aspire Capital pursuant to the Purchase Agreement and our existing stockholders would experience greater dilution.

Ourpro forma net tangible book value as of September 30, 2015 was approximately $7.3 million, or $0.24 per share. Net tangible book value per share is determined by dividing our total tangible assets, less total liabilities, by the number of shares of our common stock outstanding as of September 30, 2015.

After giving effect to the sale in this offering of 6,086,207 shares of common stock at an assumed average sale price of $0.44 per share (based on the lowest sales price of our common stock as of November 25, 2015), our pro forma as adjusted net tangible book value as of September 30, 2015December 31, 2016 would have been approximately $10.0 million,$6,793,166 or $0.27approximately $0.87 per share of Common Stock.share. This represents an immediate increase in pro forma as adjusted net tangible book value of $0.03approximately $0.22 per share to our existing stockholders and an immediate dilution in pro forma net tangible book value of $0.17approximately $0.33 per share to our new stockholders.

purchasers of Units in this offering, as illustrated by the following table:

PLAN OF DISTRIBUTIONThe discussion of dilution, and the table quantifying it, assumes no exercise of any outstanding options or warrants or the issuance of other potentially dilutive securities. The exercise of potentially dilutive securities having an exercise price less than the offering price would increase the dilutive effect to new investors.

The number of shares of Common Stock shown above to be outstanding after this offering is based on 3,786,913 shares outstanding as of December 31, 2016, and excludes shares of Common Stock issuable in connection with future option grants as well as the following as of December 31, 2016:

UNDERWRITING

Aegis Capital Corp. is acting as the representative of the underwriters and the sole book-running manager in this offering. We have entered into an underwriting agreement dated                              with the representative. Subject to the terms and conditions of the underwriting agreement, we have agreed to sell to each underwriter named below and each underwriter named below has severally and not jointly agreed to purchase from us, at the public offering price per share less the underwriting discounts and commissions set forth on the cover page of this prospectus, the number of shares of Units listed next to its name in the following table:

The underwriters are committed to purchase all the Units offered by us other than those covered by the option to purchase additional shares of Common Stock and/or Warrants described below, if they purchase any shares and/or Warrants. The obligations of the underwriters may be terminated upon the occurrence of certain events specified in the underwriting agreement. Furthermore, pursuant to the underwriting agreement, the underwriters’ obligations are subject to customary conditions and representations and warranties contained in the underwriting agreement, such as receipt by the underwriters of officers’ certificates and legal opinions.

We have agreed to indemnify the underwriters against specified liabilities, including liabilities under the Securities Act, and to contribute to payments the underwriters may be required to make in respect thereof.

The underwriters are offering the Units, subject to prior sale, when, as and if issued to and accepted by them, subject to approval of legal matters by their counsel and other conditions specified in the underwriting agreement. The underwriters reserve the right to withdraw, cancel or modify offers to the public and to reject orders in whole or in part.

Over-allotment Option. We have granted the underwriters an over-allotment option. This option, which is exercisable for up to 45 days after the date of this prospectus, permits the underwriters to purchase a maximum of ____ additional shares of Common Stock and/or Warrants to purchase a maximum of _____ shares of Common Stock (15% of the Units sold in this offering) from us to cover over-allotments, if any. If the underwriters exercise all or part of this option, they will purchase such Common Stock covered by the option at the public offering price per Unit minus once cent and the Warrants covered by the option at a price of one cent per Warrant, in each case less the underwriting discounts and commissions. If this option is beingexercised in full, the total offering price to the public will be $        and the total net proceeds, before expenses, to us will be $       .

Discounts, Commissions and Non-Accountable Expense Allowance.  The following table shows the public offering price, underwriting discount, non-accountable expense allowance and proceeds, before expenses, to us. The information assumes either no exercise or full exercise by the underwriters of their over-allotment option.

The underwriters propose to offer the Units offered by Aspire Capital,us to the Selling Stockholder. public at the public offering price per Unit set forth on the cover of this prospectus. In addition, the underwriters may offer some of the Units to other securities dealers at such price less a concession of up to $         per share. If all of the Units offered by us are not sold at the public offering price per Unit, the underwriters may change the offering price per Unit and other selling terms by means of a supplement to this prospectus.

We have also agreed to pay the representative a nonaccountable expense allowance of        % of the aggregate offering proceeds (excluding the over-allotment option), and to reimburse certain of the representative’s out of pocket expenses, including the fees of underwriters’ counsel, up to a total of $79,500.        

We estimate that the total expenses of the offering payable by us, excluding the total underwriting discounts, commissions, non-accountable expense allowance and underwriter expense reimbursement will be approximately $80,000.

Lock-Up Agreements.We have agreed with the representative that we will not offer or sell any securities for a period of 90 days from the closing date of this offering, subject to certain exceptions. In addition, all of our directors and executive officers have entered into lock up agreements with the representative prior to the commencement of this offering pursuant to which each of these persons, for a period of 90 days from the closing date of this offering, without the prior written consent of the representative, agree not to (1) offer, pledge, sell, contract to sell, sell any option or contract to purchase, purchase any option or contract to sell, grant any option, right or warrant to purchase, lend, or otherwise transfer or dispose of, directly or indirectly, any shares of our securities or any securities convertible into or exercisable or exchangeable for common shares owned or acquired on or prior to the closing date of this offering (including any common shares acquired after the closing date of this offering upon the conversion, exercise or exchange of such securities); (2) file or caused to be filed any registration statement relating to the offering of any shares of our capital shares; or (3) enter into any swap or other arrangement that transfers to another, in whole or in part, any of the economic consequences of ownership of common shares, whether any such transaction described in clause (1), (2), or (3) above is to be settled by delivery of common shares or such other securities, in cash or otherwise, except for certain exceptions and limitations.

The Common Stocklock-up period described in the preceding paragraph will be automatically extended if: (1) during the last 17 days of the lock-up period, we issue an earnings release or announce material news or a material event; or (2) prior to the expiration of the lock-up period, we announce that we will release earnings results during the 16-day period beginning on the last day of the lock-up period, in which case the restrictions described in the preceding paragraph will continue to apply until the expiration of the 18-day period beginning on the date of the earnings release.

Electronic Offer, Sale and Distribution of Securities.  A prospectus in electronic format may be sold or distributed from time to time bymade available on the Selling Stockholder directly to one or more purchasers or through brokers, dealers, or underwriters who may act solely as agents at market prices prevailing at the time of sale, at prices related to the prevailing market prices, at negotiated prices, or at fixed prices, which may be changed. The sale of the Common Stock offered by this prospectus may be effectedwebsites maintained by one or more of the following methods:

In orderunderwriters or selling group members, if any, participating in this offering and one or more of the underwriters participating in this offering may distribute prospectuses electronically. The representative may agree to comply withallocate a number of Units to underwriters and selling group members for sale to their online brokerage account holders. Internet distributions will be allocated by the securities lawsunderwriters and selling group members that will make internet distributions on the same basis as other allocations. Other than the prospectus in electronic format, the information on these websites is not part of, certain states, if applicable,nor incorporated by reference into, this prospectus or the shares may be sold only through registeredregistration statement of which this prospectus forms a part, has not been approved or licensed brokersendorsed by us or dealers. In addition,any underwriter in certain states, the shares mayits capacity as underwriter, and should not be sold unless they have been registered or qualified for sale in the state or an exemption from the registration or qualification requirement is available and complied with.relied upon by investors.

The Selling Stockholder may also sell shares ofNASDAQ Capital Market Listing.  Our Common Stock under Rule 144 promulgatedis listed on The NASDAQ Capital Market under the Securities Act,symbol “ATOS.”

Stabilization. In connection with this offering, the underwriters may engage in stabilizing transactions, over-allotment transactions, syndicate-covering transactions, penalty bids and purchases to cover positions created by short sales. Stabilizing transactions permit bids to purchase shares so long as the stabilizing bids do not exceed a specified maximum, and are engaged in for the purpose of preventing or retarding a decline in the market price of the shares while the offering is in progress.

Over-allotment transactions involve sales by the underwriters of shares in excess of the number of shares the underwriters are obligated to purchase. This creates a syndicate short position that may be either a covered short position or a naked short position. In a covered short position, the number of shares over-allotted by the underwriters is not greater than the number of shares that naked short position, the number of shares involved is greater than the number of shares in the over-allotment option. The underwriters may close out any short position by exercising their over-allotment option and/or purchasing shares in the open market.

Syndicate covering transactions involve purchases of shares in the open market after the distribution has been completed in order to cover syndicate short positions. In determining the source of shares to close out the short position, the underwriters will consider, among other things, the price of shares available for purchase in the open market as compared with the price at which they may purchase shares through exercise of the over-allotment option. If the underwriters sell more shares than could be covered by exercise of the over-allotment option and, therefore, have a naked short position, the position can be closed out only by buying shares in the open market. A naked short position is more likely to be created if available, rather than under this prospectus. In addition, the Selling Stockholder may transferunderwriters are concerned that after pricing there could be downward pressure on the price of the shares in the open market that could adversely affect investors who purchase in the offering.

Penalty bids permit the representative to reclaim a selling concession from a syndicate member when the shares originally sold by that syndicate member are purchased in stabilizing or syndicate covering transactions to cover syndicate short positions.

These stabilizing transactions, syndicate covering transactions and penalty bids may have the effect of raising or maintaining the market price of our shares or Common Stock or preventing or retarding a decline in the market price of our shares or Common Stock. As a result, the price of our Common Stock by other means not described in this prospectus.

Brokers, dealers, underwriters, or agents participating in the distributionopen market may be higher than it would otherwise be in the absence of these transactions. Neither we nor the underwriters make any representation or prediction as to the effect that the transactions described above may have on the price of our Common Stock. These transactions may be effected on The NASDAQ Capital Market, in the over-the-counter market or otherwise and, if commenced, may be discontinued at any time.

Passive market making. In connection with this offering, underwriters and selling group members may engage in passive market making transactions in our Common Stock on The NASDAQ Capital Market in accordance with Rule 103 of Regulation M under the Exchange Act, during a period before the commencement of offers or sales of the shares as agentsand extending through the completion of the distribution. A passive market maker must display its bid at a price not in excess of the highest independent bid of that security. However, if all independent bids are lowered below the passive market maker’s bid, then that bid must then be lowered when specified purchase limits are exceeded.

Certain Relationships

The underwriters and their affiliates have provided, or may in the future provide, various investment banking, commercial banking, financial advisory, brokerage, and other services to us and our affiliates for which services they have received, and may in the future receive, customary fees and expense reimbursement.

The underwriters and their affiliates may, from time to time, engage in transactions with and perform services for us in the ordinary course of their business for which they may receive compensationcustomary fees and reimbursement of expenses. In the ordinary course of their various business activities, the underwriters and their affiliates may make or hold a broad array of investments and actively trade debt and equity securities (or related derivative securities) and financial instruments (including bank loans) for their own accounts and for the accounts of their customers, and such investment and securities activities may involve securities and/or instruments of our company. The underwriters and their affiliates may also make investment recommendations and/or publish or express independent research views in respect of such securities or instruments and may at any time hold, or recommend to clients that they acquire, long and/or short positions in such securities and instruments.

Offer Restrictions Outside the United States

Other than in the form of commissions, discounts,United States, no action has been taken by us or concessions from the selling stockholder and/or purchasersunderwriters that would permit a public offering of the common stock for whom the broker-dealers may act as agent. Aspire Capital has informed us that each such broker-dealer will receive commissions from Aspire Capital which will not exceed customary brokerage commissions.

Aspire Capital is an “underwriter” within the meaning of the Securities Act.

Neither we nor Aspire Capital can presently estimate the amount of compensation that any agent will receive. We know of no existing arrangements between Aspire Capital, any other shareholder, broker, dealer, underwriter, or agent relating to the sale or distribution of the sharessecurities offered by this prospectus. At the time a particular offer of sharesprospectus in any jurisdiction where action for that purpose is made, arequired. The securities offered by this prospectus supplement, if required, willmay not be distributed that will set forth the names of any agents, underwriters,offered or dealers and any compensation from the selling stockholder, andsold, directly or indirectly, nor may this prospectus or any other required information. Pursuant to a requirement of the Financial Industry Regulatory Authority,offering material or FINRA, the maximum commission or discount and other compensation to be received by any FINRA member or independent broker-dealer shall not be greater than eight percent (8%) of the gross proceeds received by us for the sale of any securities being registered pursuant to Rule 415 under the Securities Act.

We will pay all of the expenses incident to the registration, offering, and sale of the shares to the public other than commissions or discounts of underwriters, broker-dealers, or agents. We have agreed to indemnify Aspire Capital and certain other persons against certain liabilitiesadvertisements in connection with the offeringoffer and sale of sharesany such securities be distributed or published in any jurisdiction, except under circumstances that will result in compliance with the applicable rules and regulations of our Common Stock offered hereby, including liabilities arising under the Securities Act or, if such indemnity is unavailable, to contribute amounts required to be paid in respect of such liabilities. Aspire Capital has agreed to indemnify us against liabilities under the Securities Act that may arise from certain written information furnished to us by Aspire Capital specifically for use injurisdiction. Persons into whose possession this prospectus or, if such indemnity is unavailable,comes are advised to contribute amounts requiredinform themselves about and to be paid in respect of such liabilities.

Insofar as indemnification for liabilities arising underobserve any restrictions relating to the Securities Act may be permitted to our directors, officers,offering and controlling persons, we have been advised that in the opinion of the SEC this indemnification is against public policy as expressed in the Securities Act and is therefore, unenforceable.

Aspire Capital and its affiliates have agreed not to engage in any direct or indirect short selling or hedging of our Common Stock during the term of the Purchase Agreement.

We have advised Aspire Capital that while it is engaged in a distribution of the shares included in this prospectus it is required to comply with Regulation M promulgated under the Securities Exchange Act of 1934, as amended. With certain exceptions, Regulation M precludes the Selling Stockholder, any affiliated purchasers, and any broker-dealer or other person who participates in the distribution from bidding for or purchasing, or attempting to induce any person to bid for or purchase any security which is the subject of the distribution until the entire distribution is complete. Regulation M also prohibits any bids or purchases made in order to stabilize the price of a security in connection with the distribution of that security. Allthis prospectus. This prospectus does not constitute an offer to sell or a solicitation of the foregoing may affect the marketability of the shares offered hereby this prospectus.

We may suspend the sale of shares by Aspire Capital pursuantan offer to this prospectus for certain periods of time for certain reasons, including if the prospectus is required to be supplemented or amended to include additional material information.

This offering will terminate on the date that all sharesbuy any securities offered by this prospectus have been sold by Aspire Capital.in any jurisdiction in which such an offer or a solicitation is unlawful.

DESCRIPTION OF SECURITIES TO BE REGISTERED

Our authorized capital stock consists of 75,000,000 shares of Common Stock, $0.001$0.015 par value per share, and 10,000,000 shares of preferred stock, $0.001 par value per share.

Common Stock

Holders of Common Stock are entitled to receive ratably dividends out of funds legally available, if and when declared from time to time by our Board of Directors. We have never paid any cash dividends on our Common Stock and our Board of Directors does not anticipate that we will pay cash dividends in the foreseeable future. The future payment of dividends, if any, on our Common Stock is within the discretion of the Board of Directors and will depend upon earnings, capital requirements, financial conditionconditions, and other relevant factors. Holders of Common Stock are entitled to one vote for each share held on each matter to be voted on by stockholders. There is no cumulative voting in the election of directors. In the event of liquidation, dissolution or winding up of the affairs of us, holders of Common Stock are to share in all assets remaining after the payment of liabilities and any preferential distributions payable to preferred stockholders, if any. The holders of Common Stock have no preemptive or conversion rights and are not subject to further calls or assessments. There are no redemption or sinking fund provisions applicable to the Common Stock. The rights of the holders of the Common Stock are subject to any rights that may be fixed for holders of preferred stock, if any. All of the outstanding shares of Common Stock are fully paid and non-assessable.

Warrants

The following is a summary of certain terms and conditions of the Warrants and is subject in all respects to the provisions contained in the Warrants. This summary is subject to and qualified in its entirety by the form of Warrant, which is included as an Exhibit to the registration statement to which this prospectus forms a part. You should review a copy of the form of Warrant for a complete description of the terms and conditions applicable to the Warrants.

Form. The Warrants will be issued pursuant to the terms of a warrant agency agreement between us and VStock Transfer, LLC. You should review a copy of the form of warrant agency agreement, which is included as Exhibits to the registration statement to which this prospectus forms a part, for a complete description of the terms and conditions applicable to the Warrants.

Exercisability. The Warrants are exercisable at any time after their issuance but before the date that is five years after their original issuance. The Warrants will be exercisable, at the option of each holder, in whole or in part by delivering to us a duly executed exercise notice and, at any time a registration statement registering the issuance of the shares of common stock underlying the warrants under the Securities Act is effective and available for the issuance of such shares, or an exemption from registration under the Securities Act is available for the issuance of such shares, by payment in full in immediately available funds for the number of shares of Common Stock purchased upon such exercise. If a registration statement registering the issuance of the shares of Common Stock underlying the Warrants under the Securities Act is not then effective or available, the holder may only exercise the Warrant through a cashless exercise, in which case the holder would receive upon such exercise the net number of shares of Common Stock determined according to the formula set forth in the Warrant. No fractional shares of common stock will be issued in connection with the exercise of a Warrant. In lieu of fractional shares, we will pay the holder an amount in cash equal to the fractional amount multiplied by the exercise price.

Exercise Limitation. A holder will not have the right to exercise any portion of the Warrant if the holder (together with its affiliates) would beneficially own in excess of 4.99% of the number of shares of our stock outstanding immediately after giving effect to the exercise, as such percentage ownership is determined in accordance with the terms of the Warrants. However, any holder may increase or decrease such percentage to any other percentage not in excess of 9.99% upon at least 61 days’ prior notice from the holder to us.

Exercise Price. The exercise price per share of Common Stock purchasable upon exercise of the Warrants is an amount per share of Common Stock equal to 125% of sale price of the Units included in the offering. The exercise price is subject to appropriate adjustment in the event of certain stock dividends and distributions, stock splits, stock combinations, reclassifications or similar events affecting our common stock.

Transferability. Subject to applicable laws, the Warrants may be offered for sale, sold, transferred or assigned without our consent. There is currently no trading market for the warrants and a trading market may not develop.

Exchange Listing. We do not plan on applying to list the Warrants on The NASDAQ Capital Market, any other national securities exchange or any other nationally recognized trading system.

Fundamental Transactions. In the event of a fundamental transaction, as described in the Warrants and generally including any reorganization, recapitalization or reclassification of our common stock, the sale, transfer or other disposition of all or substantially all of our properties or assets, our consolidation or merger with or into another person, the holders of the warrants will be entitled to receive, upon any subsequent exercise of the Warrants and for each share of our Common Stock that would have been issuable upon such exercise immediately prior to the occurrence of a fundamental transaction, the number of shares of Common Stock of the successor or acquiring corporation or of the Company, if it is the surviving corporation, and any additional consideration receivable as a result of such fundamental transaction by a holder of the number of shares of Common Stock for which the Warrants are exercisable immediately prior to such fundamental transaction.

Rights as a Stockholder. Except as otherwise provided in the warrants or by virtue of such holder’s ownership of shares of our Common Stock, the holder of a Warrant does not have the rights or privileges of a holder of our Common Stock, including any voting rights, until the holder exercises the Warrant.

Certificate of Incorporation

Under our certificate of incorporation, as amended, our Board of Directors, without further action by our stockholders, currently has the authority to issue up to 10,000,000 shares of preferred stock and to fix the rights (including voting rights), preferences, and privileges of these “blank check” preferred shares. Such preferred stock may have rights, including economic rights, senior to our Common Stock. As a result, the issuance of the preferred stock could have a material adverse effect on the price of our Common Stock and could make it more difficult for a third party to acquire a majority of our outstanding Common Stock.

Anti-Takeover Devices

Our certificate of incorporation and bylaws include a number of provisions that may have the effect of delaying, deferring or preventing another party from acquiring control of us and encouraging persons considering unsolicited tender offers or other unilateral takeover proposals to negotiate with our Board of Directors rather than pursue non-negotiated takeover attempts. These provisions include the items described below.

Board Composition and Filling Vacancies. In accordance with our certificate of incorporation, our Board of Directors is divided into three classes serving staggered three-year terms, with one class being elected each year. Our certificate of incorporation also provides that directors may only be removed from office for cause and only by the affirmative vote of holders of 75% or more of the outstanding shares of capital stock then entitled to vote at an election of directors. Furthermore, any vacancy on our Board of Directors, however occurring, including any vacancy resulting from an increase in the size of the board, may only be filled by the affirmative vote of a majority of our directors then in office even if less than a quorum. The classification of directors, together with the limitations on removal of directors and treatment of vacancies, has the effect of making it more difficult for stockholders to change the composition of our Board of Directors.

Undesignated Preferred Stock. Our certificate of incorporation authorizes “blank-check” preferred stock, which means that our Board of Directors has the authority to designate one or more series of preferred stock without stockholder approval. These series of preferred stock may have superior rights, preferences and privileges over our Common Stock, including dividend rights, voting rights, and liquidation preferences. The ability of our Board of Directors to issue shares of our preferred stock without stockholder approval could deter takeover offers and make it more difficult or costly for a third party to acquire us without the consent of our Board of Directors.

Section 203 of the Delaware General Corporation Law. In addition, our certificate of incorporation does not opt out of Section 203 of the Delaware General Corporation Law, which protects a corporation against an unapproved takeover by prohibiting a company from engaging in any business combination with any interested stockholder (defined as a stockholder owning more than 15% of the outstanding shares) for a period of three years from the time such stockholder became a 15% holder unless approved by our Board of Directors.

Stockholder Rights Agreement.On May 19, 2014, the Company adopted a stockholder rights agreement which provides that all stockholders of record on May 26, 2014 received a non-taxable distribution of one preferred stock purchase right for each share of our Common Stock held by such stockholder. Each right is attached to and trades with the associated share of Common Stock. The rights will become exercisable only if one of the following occurs: (1) a person becomes an “Acquiring Person” by acquiring beneficial ownership of 15% or more of our Common Stock (or, in the case of a person who beneficially owned 15% or more of our Common Stock on the date the stockholder rights agreement was executed, by acquiring beneficial ownership of additional shares representing 2.0% of our Common Stock then outstanding (excluding compensatory arrangements)),; or (2) a person commences a tender offer that, if consummated, would result in such person becoming an Acquiring Person. If a person becomes an Acquiring Person, each right will entitle the holder, other than the Acquiring Person and certain related parties, to purchase a number of shares of our Common Stock with a market value that equals twice the exercise price of the right. The initial exercise price of each right is $15.00, so each holder (other than the Acquiring Person and certain related parties) exercising a right would be entitled to receive $30.00 worth of our Common Stock. If the Company is acquired in a merger or similar business combination transaction at any time after a person has become an Acquiring Person, each holder of a right (other than the Acquiring Person and certain related parties) will be entitled to purchase a similar amount of stock of the acquiring entity.

Transfer Agent and Registrar

We have appointed VStock Transfer, LLC, 18 Lafayette Place, Woodmere, New York 11598 (Telephone: (212) 828-8436; Facsimile (646) 536-3179) as our transfer agent and registrar.registrar for our Common Stock and Warrants.

Listing

Our Common Stock is listed on theThe NASDAQ Capital Market under the symbol “ATOS”.“ATOS.”

DISCLOSURE OF COMMISSION POSITION ON INDEMNIFICATION FOR SECURITIES ACT LIABILITIES

Insofar as indemnification for liabilities arising under the Securities Act may be permitted to directors, officers, and controlling persons of the registrant pursuant to the foregoing provisions, or otherwise, the registrant has been advised that in the opinion of the Securities and Exchange Commission such indemnification is against public policy as expressed in the Securities Act and is, therefore, unenforceable.

LEGAL MATTERS

Certain legal matters relating to the validity of the Common StockUnits offered by this prospectus will be passed upon for us by Gibson, Dunn & Crutcher LLP, San Francisco, California. Certain legal matters will passed upon for the underwriters by Ellenoff Grossman & Schole LLP, New York, New York.

EXPERTS

The consolidated financial statements as of December 31, 20142016 and 2015 and for each of the year thentwo years in the period ended December 31, 2016 incorporated by reference in this Prospectus have been so included in reliance on the report of BDO USA, LLP, an independent registered public accounting firm (the report on the consolidated financial statements contains an explanatory paragraph regarding the Company'sCompany’s ability to continue as a going concern) which is incorporated by reference in the Prospectus, given on the authority of said firm as experts in auditing and accounting. KCCW Accountancy Corp., an independent PCAOB registered public accounting firm, has audited the Company’s consolidated balance sheets as of December 31, 2013 and the related consolidated statements of operations, stockholders’ equity, and cash flows for the year then ended, which are incorporated by reference in this prospectus. The consolidated financial statements are included in reliance on the report of KCCW Accountancy Corp., given their authority as experts in accounting and auditing.

WHERE YOU CAN FIND ADDITIONAL INFORMATION

We are required to file annual, quarterly and special reports, proxy statements and other information with the SEC. You may read and copy any document filed by us at the SEC’s Public Reference Room at 100 F Street, N.E., Washington, D.C. 20549. Please call the SEC at 1-800-SEC-0330 for further information on the public reference room. Our filings with the SEC are also available to the public at the SEC’s Internet web site athttp://www.sec.gov.

We have filed a registration statement, of which this prospectus is a part, covering the securities offered hereby. As allowed by SEC rules, this prospectus does not include all of the information contained in the registration statement and the included exhibits, financial statements and schedules. You are referred to the registration statement, the included exhibits, financial statements and schedules for further information. This prospectus is qualified in its entirety by such other information.

INCORPORATION OF CERTAIN INFORMATION BY REFERENCE

The SEC allows us to “incorporate by reference” information from other documents that we file with it, which means that we can disclose important information to you by referring you to those documents. The information incorporated by reference is considered to be part of this prospectus. Information in this prospectus supersedes information incorporated by reference that we filed with the SEC prior to the date of this prospectus.

We incorporate by reference into this prospectus and the registration statement of which this prospectus is a part the information or documents listed below that we have filed with the SEC (Commission File No. 001-35610):