Addendum to the Employment

EXHIBIT 10.32

March 30, 2005

Dr. Nassim Usman

2129 Night Sky Lane

Lafayette, CO 80026

Dear Nassim:

This letter shall serve as an addendum to the terms of employment letter with Sirna Therapeutics, Inc. (the “Company”) dated February 11, 2003, as amended by letter dated April 14, 2003 (the “Employment Agreement”). You hereby acknowledge your acceptance of and agreement with this addendum (the “Addendum”) on such terms as detailed below.

Capitalized terms used herein and not otherwise defined herein shall have the meanings ascribed to them in the Employment Agreement.

1. Separation from Employment. You shall hereby be deemed to have separated from your position of employment with the Company with a severance date of March 16, 2005 (the “Severance Date”). Commencing on the date hereof, you shall cease all activities on behalf of or involving the Company (unless the Company otherwise requests in writing), including, without limitation, making any presentations and attending meetings for or on behalf of the Company and you will transition (unless the Company otherwise requests in writing) (a) all business development activities to Ms. Rebecca Robison or Dr. Bharat Chowrira of the Company, and (b) all speaking engagements and other presentation responsibilities to Dr. Barry Polisky. Within five (5) business days after the Severance Date, the Company will pay to you all accrued but unpaid salary and all accrued but unused vacation time (203 hours), less all applicable deductions and withholding. You are entitled to these payments regardless of whether you sign this Addendum.

2. Stock Option Vesting. For the purposes of Section 3(d)(iii) of the Employment Agreement, your unvested Company stock options will continue to vest, on a monthly basis, as set forth under Section 3(d)(iii) of the Employment Agreement, except that such vesting shall terminate upon the earlier to occur of (i) your obtaining comparable new employment during the Severance Period and (ii) termination or expiration of the Severance Period as provided herein (either, a “Vesting Termination Date”). Upon the termination of such vesting period, all of your unvested and vested options shall cease vesting and you shall have ninety (90) days to exercise any or all of your vested stock options Following the Vesting Termination Date, the Company shall extend the option exercise period so that you may exercise any or all of your vested stock

options at any time until ten (10) days after the earliest of (x) the expiration or termination (for any reason) of the Consulting Period (as defined below), (y) the date of termination of this Addendum pursuant to Section 16 hereof and (z) notification on or before December 15, 2005 that the options should expire and no longer be exercisable; and thereafter all of your unexercised vested stock options will terminate. Under applicable tax laws, your vested stock options that have not been exercised three (3) months following the Severance Date shall be treated as non-qualified stock options. You acknowledge that the Company has not given you any tax advice and that you have relied solely upon your own tax advisors relating to any and all tax matters relating to this Agreement and the subject matter hereof.

3. Severance Payment. For the purposes of severance payment set forth under Section 3(d)(iv) of the Employment Agreement, the Company will pay you in monthly installments (two pay checks per month in accordance with Company’s standard payroll practices) during the Severance Period (i) one-twelfth (1/12) of your 2004 Base Salary, and (ii) one-twelfth (1/12) of $25,400 (which is your annual target bonus equal to the lesser of your full annual target bonus for the 2005 calendar year and the average of your actual annual bonuses for the previous two (2) calendar years); provided that in the event you obtain other employment during the Severance Period, your severance payments thereafter shall be reduced on a prospective basis (to not less than 0) in the amount of cash compensation received by you during the remainder of such Severance Period. For the avoidance of doubt, in the event this Addendum is terminated pursuant to Section 17 hereof, then all severance payments shall cease immediately. For purposes of this Addendum, the “Severance Period” shall mean the period commencing on the Severance Date and ending on the earlier of (x) December 15, 2005 and (y) the date of termination of this Addendum pursuant to Section 17 hereof.

4. Consulting Arrangement. If, and only if, the exercise period for the options extends beyond December 15, 2005, then commencing on the date immediately following the end of the Severance Period and ending on the earlier of (i) the third anniversary of such date, (ii) the date of termination of this Addendum pursuant to Section 17 hereof, and (iii) mutual agreement of the parties (the “Consulting Period”), the Company agrees to engage you as a consultant to the Company, and you agree to serve as a consultant to the Company, upon the terms and conditions set forth in this Section 4. You will report to the President and Chief Executive Officer. You agree to present on behalf of the Company at conferences and symposia, and perform such other consulting services as may be requested by the Company to the best of your ability, and generally promote the Company, its technology and actual and proposed products and services in a positive manner. You will be paid a consulting fee of $10,000 per year, payable in equal monthly installments, being an aggregate amount of $30,000 for such consulting fees payable by the Company to you with respect to the Consulting Period. You will be entitled to an additional fee of $2,500 per day if and when requested by the Company to present at a conference or symposium on its behalf, together with reimbursement for your actual, reasonable out-of-pocket expenses incurred by you in the performance of such consulting services, including economy class air travel and hotel, local transportation and similar expenses; provided that (x) the Company’s prior written approval shall be required for (1) any travel expenses and (2) any expenses in excess of $100 in the

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aggregate incurred in any month, and (y) you shall provide to the Company documentation in substantiation of all expenses when seeking reimbursement and otherwise comply with the Company’s expense reimbursement policy and procedures as in effect from time to time. During the Consulting Period, you will be an independent contractor, and not an employee, of the Company and shall not have any power or authority to create any obligation in the name of or on behalf of the Company. The Company will not withhold from any consulting fees payable to you any funds for federal, or state or local income taxes, social security, disability insurance contributions or any other taxes or contributions paid by employers for employees. The Company shall provide you with an IRS Form 1099 with respect to your consulting fees for federal income tax purposes, and you assume full and exclusive responsibility and liability for filing any required federal, state and local income tax forms and paying any and all federal, state and local taxes as and when due (including, if applicable, quarterly estimated taxes with respect to any consulting fees paid to you).

5. Health Care Coverage. For the purposes of Health Care Coverage provided under Section 3(d)(v) of the Employment Agreement, the Company agrees to pay premium costs relating to maintaining Health Care Coverage under COBRA for you and your dependents for the period of eighteen (18) months after the Severance Date or such earlier date as may be allowed by law; provided that such Health Care Coverage shall terminate upon your obtaining comparable Health Care Coverage from a future employer (after taking into account any waiting periods for such coverage to become effective). For the avoidance of doubt, you will be responsible at all times for all deductibles, co-payments, “buy-up” options and uncovered or unreimbursed medical expenses under such Health Care Coverage for you and your dependents.

6. Outplacement Assistance. The Company agrees to provide you outplacement assistance with a third party outplacement service firm for a six (6) month period, at a cost to be paid by the Company to such outplacement service firm not to exceed $4,000. Any additional costs shall be your sole responsibility.

7. Waiver and Release of Claims.

        (a) In exchange for the consideration provided under this Addendum, you freely and voluntarily agree to the Waiver and Release set forth below.

        (b) You warrant and represent that you have not previously assigned or transferred any claim released in this Addendum, and that you shall defend, indemnify and hold harmless the Company from and against any claim (including the payment of attorneys’ fees and costs actually incurred whether or not litigation is commenced) based on or arising out of any such assignment or transfer made, purported or claimed

        (c) Except for the obligations created under this Addendum (which includes obligations which arise from your Employment Agreement an