PERSEID THERAPEUTICS LLC CO-SALE AGREEMENT

Exhibit 2.1.6

PERSEID THERAPEUTICS LLC

CO-SALE AGREEMENT

September 18, 2009

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PERSEID THERAPEUTICS LLC

CO-SALE AGREEMENT

This Co-Sale Agreement (this “ Agreement ”) is dated as of September 18, 2009, and is between Perseid Therapeutics LLC, a Delaware limited liability company (the “ Company ”), Maxygen, Inc., a Delaware corporation (“ Maxygen ”), and Astellas Bio Inc., a Delaware corporation (“ Bio ”) (each of Bio and Maxygen an “ Investor ,” and collectively, the “ Investors ”). All capitalized terms used and not defined herein shall have such meanings as set forth in the Master Joint Venture Agreement by and between Maxygen, Astellas Pharma Inc. and Bio dated as of June 30, 2009 (the “ Master Joint Venture Agreement ”).

RECITALS

The Investors are parties to the Series A and Series B Preferred Unit Purchase Agreement of even date herewith, between the Company and the Investors (the “ Purchase Agreement ”), and it is a condition to the closing of the sale of the Series A Preferred Units and Series B Preferred Units to the Investors that the Investors and the Company execute and deliver this Agreement.

The parties therefore agree as follows:

SECTION 1

DEFINITIONS

1.1 Certain Definitions . For purposes of this Agreement, the following terms have the following meanings:

(a) “ Buy-Out Option ” has such meaning as set forth in Section 5.1 of the Investors’ Rights Agreement.

(b) “ Common Units ” means the common units of the Company.

(c) “ Change of Control ” means (a) the acquisition of the Company by another entity by means of any transaction or series of related transactions to which the Company is party (including, without limitation, any stock acquisition, reorganization, merger or consolidation) other than a transaction or series of transactions in which the holders of the voting securities of the Company outstanding immediately prior to such transaction continue to retain (either by such voting securities remaining outstanding or by such voting securities being converted into voting securities of the surviving entity), as a result of units in the Company held by such holders prior to such transaction, at least fifty percent (50%) of the total voting power represented by the voting securities of the Company or such surviving entity outstanding immediately after such transaction or series of transactions, excluding any bona fide sale of units for capital raising purposes, or (b) the sale, lease, exclusive license or other conveyance of all or substantially all of the assets of the Company.

(d) “ Convertible Securities ” means all then outstanding options, warrants, rights, convertible notes, preferred units or other securities of the Company, directly or indirectly convertible into, or exercisable for, Common Units.

(e) “ Days ” means calendar days; provided that if any day on which a period specified in this Agreement would otherwise terminate falls on a weekend or a federal holiday, the term “ day ” shall mean the next Business Day.

(f) “ Preferred Units ” means the Series A Preferred Units and Series B Preferred Units of the Company.

(g) “ Remaining Investor ” means an Investor that is not proposing to Transfer Seller Units.

(h) “ Rights of Co-Sale ” means the right of co-sale in Section 3 provided to the Remaining Investor.

(i) “ Seller ” means an Investor proposing to Transfer Seller Units.

(j) “ Seller Units ” means all Common Units, Preferred Units and Convertible Securities of the Company owned as of the date hereof or hereafter acquired by an Investor, as adjusted for any unit splits, unit dividends, combinations, subdivisions, recapitalizations and the like.

(k) “ Series A Preferred Units ” means all of the Series A Preferred Units issued pursuant to the Purchase Agreement.

(l) “ Series B Preferred Units ” means all of the Series B Preferred Units issued pursuant to the Purchase Agreement.

(m) “ Transfer ,” “ Transferring ,” “ Transferred ,” or words of similar import, mean and include any sale, assignment, encumbrance, hypothecation, pledge, conveyance in trust, gift, transfer by bequest, devise or descent, or other transfer or disposition of any kind, including but not limited to transfers to receivers, levying creditors, trustees or receivers in bankruptcy proceedings or general assignees for the benefit of creditors, whether voluntary or by operation of law, directly or indirectly, except:

(i) any bona fide pledge made pursuant to a bona fide loan transaction that creates a mere security interest, if the pledgee executes a counterpart copy of this Agreement and becomes bound thereby as a Seller in the event that and to the extent that such pledgee ever acquires ownership of such Units; and

(ii) any transfer (either by operation of law or otherwise) made by either of Bio or Maxygen to (i) an entity that acquires all or substantially all of the business or assets of Bio or Maxygen (including, in the case of Maxygen all or substantially all of the assets of Maxygen other than the Buy-Out Units), as the case may be, whether by merger, reorganization, acquisition, asset sale or otherwise, or (ii) an Affiliate of Bio or Maxygen, as the case may be (including, for the avoidance of doubt and without limitation, a liquidating trust or similar entity), in each case subject to the agreement in writing of such transferee to be subject to the terms and conditions of this Agreement and, prior to the Option Expiration Date (as defined in Section 5.1(a) of the Investors’ Rights Agreement), the Investors’ Rights Agreement, the Voting Agreement and the LLC Agreement.

If a Seller plans to make any of the above excepted transfers, then, prior to transferring its Seller Units, the Seller shall deliver to the Company a written notice stating: (A) Seller’s bona fide intention to make an excepted transfer of its Seller Units; (B) the name, address and phone number of each proposed transferee; (C) the aggregate number of Seller Units to be transferred to each proposed transferee; and (D) the section in this agreement upon which Seller is relying in making an excepted transfer.

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SECTION 2

RESTRICTIONS ON TRANSFER

2.1 General . Prior to the Option Expiration Date, no Party may Transfer their Units or Conversion Units (except as in accordance with Section 5.1 of the Investors’ Rights Agreement or in a Permitted Transfer (as defined in the Investors’ Rights Agreement)) pursuant to Sections 5.1 and 6.4 of the Investors’ Rights Agreement. On or after the Option Expiration Date (as defined the Investors’ Rights Agreement), before a Seller may Transfer any Seller Units, Seller must comply with the provisions of Section 2.2 and Section 3 hereof. Each Investor represents and warrants that it is the sole legal and beneficial owner of its Seller Units and, subject to any restrictions imposed under the Company’s certificate of formation, as filed on September 8, 2009, and as amended from time to time (the “ Certificate ”) or the Company’s limited liability company agreement, as amended from time to time (the “ LLC Agreement ”), or under any restricted units purchase agreement with the Company, that no other person or entity has any interest (other than a community property interest) in such units. Each of Bio and Maxygen acknowledges and agrees that it is subject to certain restrictions on the Transfer of its Seller Units prior to the Option Expiration Date pursuant to Section 5 of the Investors’ Rights Agreement and that such restrictions are in lieu of the restrictions on transfer set forth in this Agreement, and that none of the restrictions on transfer set forth in this Agreement shall apply, with respect to any Transfer of Seller Units prior to the Option Expiration Date.

2.2 Notice of Proposed Transfer . On or after the Option Expiration Date, prior to Seller Transferring any of its Seller Units, Seller shall deliver to the Company and the Remaining Investor a written notice (the “ Transfer Notice ”) in substantially the form attached hereto as Exhibit B, stating: (i) Seller’s bona fide intention to Transfer such Seller Units; (ii) the name, address and phone number of each proposed purchaser or other transferee (each, a “ Proposed Transferee ”); (iii) the aggregate number of Seller Units proposed to be Transferred to each Proposed Transferee (the “ Offered Units ”); (iv) the bona fide cash price or, in reasonable detail, other consideration for which Seller proposes to Transfer the Offered Units (the “ Offered Price ”); and (v) the Remaining Investor’s right to exercise its Right of Co-Sale with respect to the Offered Units. Attached to such Transfer Notice shall be a copy of the purchase agreement (the “ Transfer Agreement ”) pursuant to which the Seller intends to Transfer its Seller Units. If the Transfer Agreement is not available at the time that the Transfer Notice is sent to the Company and the Remaining Investor, then the Seller agrees to provide the Transfer Agreement to the Company and the Remaining Investor promptly once a reasonably final draft thereof is available and in any event not less than five (5) days prior to the conclusion of the Initial Exercise Period.

SECTION 3

RIGHT OF CO-SALE

3.1 Exercise by the Remaining Investor .

(a) Subject to the limitations of this Section 3, the Remaining Investor shall have the right to participate in such sale of the Offered Units on the same terms and conditions as specified in the Transfer Agreement an