Exhibit 10.24December 13, 2005Steven MayerDear Steve:Confirming the Agreement reached with the Board of Directors regarding your separation fromemployment with Human Genome Sciences, Inc. (“HGS” or “Company”) and your pursuit of a newprofessional opportunity with CoGenesys, HGS offers you the following terms contingent upon youragreement with the terms set forth below:1) You have elected to resign from your position as Chief Financial Officer effectiveDecember 12, 2005, but you will remain employed by HGS through December 31, 2005. You willbe paid your salary through December 31, 2005. Effective January 1, 2006, youwill begin to serve as a consultant to HGS. During this period, you will make yourselfreasonably available to provide advice on real estate transactions, financial transactions,and assistance related to transition of CFO duties, it being understood and agreed that (i)such services shall be provided at times and places reasonably agreeable to the parties,taking into account your business and personal schedule and (ii) it is anticipated that youwill dedicate approximately 20% of your time each month to consulting on behalf of HGS.2) In exchange for your agreement to provide consulting services pursuant to Section 1hereof, HGS agrees to pay you commencing January 1, 2005 bi-weekly through June 30, 2006 asum equivalent to two weeks of your current annual base salary. No withholdings ordeductions will be taken or made from any monies paid to you for consulting services and HGSwill issue a Form 1099 at the end of 2006 reflecting all payments made to you pursuanthereto. In addition, you will be eligible to receive a bonus award for 2005 calculated tobe 40% of your estimated 2005 earnings. This bonus will amount to $137,523.00 and will bepaid on or before December 31, 2005. In addition: a) Following your termination date, at your election, you will be permittedto continue to participate in the HGS medical, dental, and vision care plans underCOBRA. Your rights under COBRA begin as of January 1, 2006 and will be communicatedseparately through our third party administrator, Conexis. The Company willreimburse you for the cost of COBRA continuation coverage through June 30,
 

 
   2006. HGS will reimburse you either monthly or in total at the end of the 6 monthCOBRA period for which reimbursement will be made.  b) Your participation in the 401(k) Plan, including the HGS match and thelife and ADD insurance plans terminate effective December 31, 2005. Any vestedrights that you currently have in the 401(k) Plan shall not be affected by thisAgreement.  c) Your vacation leave accrual ceases effective December 31, 2005. Paymentfor all accrued, unused vacation leave will be made in the next payroll run.  d) You will be credited, as of December 31, 2005 with 6 months of additionalvesting service under all outstanding equity awards made to you under the Company’sStock Incentive Plans so that such equity awards will have a vested status as ofDecember 31, 2005 as that which they would otherwise have had on June 30, 2006,under each applicable vesting schedule had your employment with the Companycontinued uninterrupted through June 30, 2006.  e) Your right to exercise all outstanding stock options, to the extent vested(including the acceleration of vesting described in the preceding paragraph), willbe extended to the earlier of the original ten-year-term expiration date of theapplicable option agreement or December 31, 2006. Any vested stock options that arenot exercised by 5:00 p.m. Eastern Time on the earlier of the original ten-year-termexpiration date of the applicable option agreement or December 31, 2006, shallexpire at that time and shall no longer be exercisable thereafter.  f) You will be reimbursed for all ordinary and necessary reasonable businessrelated expenses incurred by you prior to December 31, 2005. You must submit yourrequest for reimbursement for these expenses, accompanied by proper documentation,to me on or before January 10, 2006.  g) You acknowledge and agree that the terms set forth above includecompensation and benefits to which you are not otherwise entitled. Furthermore, youacknowledge that, except as expressly set forth herein, you are entitled to no otheror further compensation, remuneration or benefits from HGS.3) In exchange and as consideration for the promises and undertakings set forth above, youagree as follows: a) You acknowledge that by reason of your position with the Company you havebeen given access to trade secrets and confidential information. You agree that youwill comply with the terms of the Employee Confidential Information, Invention andNon-Competition Agreement you have signed with the Company, the terms of which areincorporated herein, and agree to return to the Company and, except as set forth onExhibit A, shall not take, copy or disclose in any form or manner, any tangible workproduct belonging to HGS.
 

 
   Except as set forth on Exhibit A, you also agree that you will not remove anytangible property of HGS from the Company’s premises. Not later than December 31,2005 and except as set forth on Exhibit A, you will return to HGS all Companyproperty in your possession or control. You agree not to delete any information,files or any other materials from HGS’ computer systems or any computer owned byHGSother than in the ordinary course. You further understand pursuant toyour Employee Confidential Information, Invention and Non-Competition Agreement withthe Company that you have assigned and are obligated to assign any inventions made byyou at the Company and are further obligated to execute patent documents related tosuch inventions and hereby agree to execute all such documents.  b) You recognize the need for HGS to maintain continued harmoniousrelationships with its workforce, the public, the scientific community, theregulatory authorities, and the business community. Therefore, you agree that youwill not make any public statement or any private statement to any HGS employee,customer, potential customer, investor, potential investor, business analyst or anyother person or entity doing business or likely to do business with HGS, disparagingHGS or criticizing its operations, personnel, or employment practices. HGS agreesthat neither it nor any member of its Board of Directors or any officer will makeany public statement or private statement to any business analyst, or person orentity actually or potentionally doing business or likely to do business with you orany entity with which you are associated (whether as a customer, investor, employeeor consultant) disparaging you. Both parties agree that neither he, it or they willmake or publish any communication that reflects adversely upon the reputation of theother party.  c) You hereby forever release, waive, discharge and covenant not to sue HGS,its directors, officers, agents and employees (in their individual and/orrepresentative capacities), affiliates, subsidiaries, successors and assigns(collectively, the “Releasees”) relating to any claims, demands, charges, suits andcauses of action, whether known or unknown, up to the effective date of thisAgreement that are in any way whatever related to your employment by, or terminationof employment with, HGS and its affiliates, which you have or may have against anyof the Releasees to the greatest extent permitted by law; provided that this releaseshall not apply to, and you shall not be considered to have released any, (i) claimsfor accrued compensation and benefits; or (ii) claims arisingunder this Agreement, **THIS IS INTENDED TO BE A GENERAL RELEASE**.    You expressly agree not to join in, encourage or voluntarily assist others in thefiling or pursuit of a lawsuit or claim against the Releasees that is related to youremployment by, or termination of employment with, HGS, provided that nothing in thisparagraph shall preclude you from complying with your legal obligations as describedin Paragraph 3(c) above. You agree not to provide any aid or assistance of any type,other than testimony required by subpoena, to any person or entity
 

 
   who has filed or who seeks to file or prosecute any grievance, claim, complaint orlawsuit against the Releasees. You further agree that in the event that you areasked or required by subpoena to provide testimony to any person or entity who hasfiled or who seeks to file or prosecute any grievance, claim, charge, complaint orlawsuit against the Releasees, to provide written notice of such request or subpoenato HGS’ General Counsel, James H. Davis, in writing, at 14200 Shady Grove Road,Rockville, MD 20850, within five business days of the date you are aware that youhave received such request or subpoena. If any individual or entity brings a lawsuitor other claim against any of the Releasees on behalf of you or a class of which youare a part, involving any matter released hereby, you agree not to seek or accept anydamages or other personal relief in such proceeding.    You agree that, in the event the Releasees are or becomes a party or witness to anyactual or threatened legal proceeding regarding any matter which arose or occurredduring the term of your employment with HGS, you shall make yourself reasonablyavailable (taking into account your personal and business schedule) to and cooperatewith HGS and its counsel in such proceedings.    The foregoing release, waiver and covenant not to sue pertains to, but is not limitedto, claims arising under: federal, state and local statutes, ordinances andregulations prohibiting employment discrimination (including, but not limited to,claims arising under the Age Discrimination in Employment Act of 1967, as amended;the Older Worker Benefit Protection Act (“OWBPA”); Title VII of the Civil Rights Act,as amended; the Fair Labor Standards Act, and the Americans with Disabilities Act);the Employee Retirement Income Security Act of 1974, as amended; and any and allother federal, state and local statutes, cases, authorities or laws (including commonlaw) providing a cause of action that can be the subject of a release underapplicable law. Nothing in this release shall be construed to waive any claims thatcannot be waived as a matter of law.  d) You agree that (i) your continued failure to materially abide by theconsulting provisions contained in paragraph 1 and (ii) your intentional or recklessfailure to comply with the nondisparagement provision of Paragraph 3(b) above, shallrelease HGS from honoring any outstanding obligations owed to you under thisAgreement, among whatever other relief may be available under the terms of thisAgreement and at law or in equity.  e) In executing this Agreement, you affirm that you are competent andunderstand and accept the nature, terms, and scope of this Agreement as fully andtotally resolving any differences and disputes between you and HGS concerning thecircumstances of your employment and your termination from employment. You furtheraffirm that your decision to execute this Agreement is entirely voluntary, and thatyou have done so without any pressure or undue influence by HGS or itsrepresentatives or agents.
 

 
4) The parties mutually understand and agree that by executing this Agreement, the Releaseesare not admitting any violation of your rights under statutory or common law or any contractor Agreement entered into with HGS.5) If any provision of this Agreement is found to be invalid, unenforceable or void for anyreason, such provision shall be severed from the Agreement and shall not affect the validityor enforceability of the remaining provisions. This Agreement shall be deemed to have beenmade and entered into in Maryland and shall in all respects be interpreted, enforced andgoverned by the laws of Maryland, without giving effect to the conflicts of laws provisionsthereof.6) This Agreement shall be binding upon the parties hereto and their respective heirs,personal representatives, successors, and assigns.7) Any controversy or claim arising out of, or relating to, this Agreement shall first bediscussed informally between you and the Company in an attempt to resolve the matter. Ifyou and the Company are unable to resolve the matter, then any controversy or claim arisingout of, or relating to, this Agreement shall be submitted to mandatory private mediation andthen, if necessary, to final and binding private arbitration in accordance with the rulesand procedures of the American Arbitration Association National Rules for the Resolution ofEmployment Disputes (“AAA Rules”). It is agreed that the procedures set forth in thisparagraph will be the exclusive, final, and binding means for the resolution of any disputesarising out of or relating to this Agreement, except as provided below in Paragraph 8. Itis further agreed that any proceedings initiated pursuant to this provision will be broughtin Washington, D.C. The parties shall share the costs of the mediation/arbitration process,with the Company paying three-quarters and you paying one-quarter. Each party shall beresponsible for paying their own attorneys’ fees incurred with regard to suchmediation/arbitration.8) You acknowledge and agree that a breach by you of any of the provisions of Paragraph 3 ofthis Agreement will cause immediate and irreparable injury to the Company, for which injurythere is no adequate remedy at law. Accordingly, Paragraph 7 of this Agreementnotwithstanding, you agree that in the event of such breach or threatened breach, theCompany will be entitled to an immediate injunction by a court of competent jurisdiction toprevent and restrain such breach, with the losing party being required to pay the costs andattorneys’ fees incurred by the other in bringing or defending, as applicable, any courtaction or arbitration claim relating to such actual or threatened breach of said paragraphs.You expressly agree to waive the requirement for HGS to post a bond in any such injunctionaction and you expressly agree to submit to the jurisdiction of the courts of the State ofMaryland for any injunction proceeding brought pursuant to this provision.9) You shall be indemnified for actions or inactions occurring during your employment withHGS and its affiliates to the fullest extent permitted by Delaware law and HGS’s by-laws asin effect on the date hereof.
 

 
10) You and HGS agree that this Agreement, consisting of seven (7) pages, supersedes anyprevious agreements between the parties, provided that the Employee ConfidentialInformation, Invention and Non-Competition Agreement, and any and all other confidentiality,intellectual property, and non-compete agreements signed by you and HGS shall survive thisAgreement and shall remain in full force and effect after your separation from HGS and areincorporated herein. You agree that in executing this Agreement, you do not rely on anystatement or promise by HGS or any of its agents or employees, other than what is written inthis Agreement. This Agreement may be modified only by a written document signed by bothyou and the CEO of HGS. You acknowledge that this Agreement is a full andaccurate embodiment of the understanding between the parties.11) The parties, intending to be legally bound by this Agreement, each execute it withoutcoercion and with knowledge of the nature and consequences of its terms and conditions.12) You acknowledge that you have been given the opportunity to consider this Agreement for upto twenty-one (21) days. You also acknowledge that you have been advised in writing toconsult an attorney about this Agreement and that you have had a fair and full opportunityto do so. Your signature on this Agreement prior to the expiration of this period evidencesyour intent to knowingly and voluntarily waive this review period.13) You may revoke this Agreement within seven (7) days, following its execution by notifyingthe General Counsel of HGS, James H. Davis, in writing, at 14200 Shady Grove Road,Rockville, MD 20850. If the Agreement is revoked, it is null and void. In any event, theAgreement does not become effective until the seven (7) day revocation period expires.After that time, if there has been no written revocation, this Agreement shall be fullyeffective, enforceable and irrevocable.14) You understand that your obligation to comply with the Employee Confidential Information,Invention and Non-Competition Agreement that you signed upon commencing employment with HGSwill continue, regardless of whether or not you accept this Agreement.
 

 
If you agree with these terms, please sign this Agreement below and return it to me by January 3,2006. Your signature will confirm that you are entering into this Agreement voluntarily and with afull understanding of all of the above terms. No changes in this Agreement will be valid unless inwriting and signed by both parties.Sincerely,/s/ Susan Bateson McKay