Exhibit 10.21











 


 


 





December 17, 2013


 


Michael Houston, Ph.D.


22626 NE Inglewood Hill Road


Sammamish, WA 98074


 


Dear Mike,


 


PhaseRx Inc. is pleased to extend the following offer to you
for the position of Vice President, Therapeutics Development at PhaseRx, Inc., (the “Company”) reporting directly to
me. Your start date will be January 1, 2013.


 


Your starting salary will be $100,000 annually, paid semi monthly
($4,166.66), less all legally required withholdings. This position is classified as exempt. You will be eligible to receive a performance
evaluation for the review cycle ending December 31, 2014, at which time your compensation will be reviewed.


 


Subject to approval by the Company’s Board of Directors,
you will be granted an option to purchase 300,000 shares of the Company’s Common Stock at an exercise price equal to the
fair market value per share of the Common Stock on the date the Board of Directors approves the option grant. The shares will vest
over a four year period with vesting commencing the first anniversary of your employment.


 


Your compensation includes participation in all benefits offered
generally to PhaseRx employees. Benefits currently offered include medical, dental, vision insurance, an employee assistance and
resource & referral services program (EAP) for you and eligible family members; company paid life and accidental death &
dismemberment insurance, company paid short and long term disability insurance, a transportation benefit which provides a bus pass
or subsidized parking with a bus pass for occasional use, and participation in a 401k retirement savings plan. Effective on your
date of hire, you will begin accruing vacation and sick leave each pay period up to a maximum of 15 days vacation and 10 days sick
leave, per year. In addition, employees receive 12 paid holidays annually, including a paid holiday break from December 26 - December
31.


 


Our policies and practices, including the compensation and benefits
we provide, are subject to changes and exceptions without prior notice, at the Company’s discretion.


 


This offer is contingent upon your
ability to provide to the Company documentary evidence of your identity and eligibility for employment in the United States in
accordance with immigration regulations relating to employment eligibility. Such documentation must be provided within 3 


business days of the effective date of your employment, or your
employment relationship with the Company may be terminated.


 


 



     


     


    




 


 


 


The company reserves the right to conduct background investigations
and/or reference checks on all its potential employees. Your job offer is therefore contingent upon a clearance of such a background
investigation and/or reference check.


 


Please note that acceptance of this offer does not create a
contract of continuing employment with PhaseRx. Employment with the company is on an at-will basis, meaning that the employee or
the company may terminate the employment relationship at any time and for any reason not expressly prohibited by law.


 


As a condition of this offer of employment, you will be required
to complete, sign and return the


Company’s standard form of At-Will Employee Agreement
(the “Employee Agreement”), which is attached hereto. Please sign and return this agreement on your first day of employment.


 


This offer letter and the Employee Agreement, when signed by
you, set forth the terms of your employment with the Company and supersede any and all prior representations and agreements whether
written or oral. In the event of a conflict between the terms and provisions of this offer letter and the Employee Agreement, the
terms and provisions of the Employee Agreement will control. Any amendment of this offer letter or any waiver of a right under
this offer letter must be in writing, signed by you and an officer of the Company. Washington law will govern this offer letter.


 


If you are in agreement with these terms please sign two copies
of this letter and return one to me by December 19, 2013 to acknowledge your acceptance.


 


I am very excited about the prospect of welcoming you to the
PhaseRx team and I am confident that you will be able to significantly contribute to the success of our company.


 


Sincerely,


 


/s/ Robert W. Overell


 


Robert W. Overell, Ph.D.


President and CEO


 


 


	/s/ Michael Houston	 	12/17/13	 
	Michael Houston, Ph.D	 	Date