Viracta Therapeutics (VIRX) 10-Q 2016 Q1 Quarterly report

Exhibit 32.1

CERTIFICATION OF CHIEF EXECUTIVE OFFICER AND CHIEF FINANCIAL OFFICER

Pursuant to the requirement set forth in Rule 13a-14(b) of the Securities Exchange Act of 1934, as amended, (the “Exchange Act”) and Section 1350 of Chapter 63 of Title 18 of the United States Code (18 U.S.C. §1350), Daniel N. Swisher, Jr., Chief Executive Officer and President, and Eric H. Bjerkholt, Executive Vice President, Corporate Development and Finance and Chief Financial Officer, of Sunesis Pharmaceuticals, Inc. (the “Company”), each hereby certifies that, to the best of his knowledge:

1. The Company’s Quarterly Report on Form 10-Q for the period ended March 31, 2016 (the “Periodic Report”), to which this Certification is attached as Exhibit 32.1, fully complies with the requirements of Section 13(a) or Section 15(d) of the Exchange Act; and

2. The information contained in the Periodic Report fairly presents, in all material respects, the financial condition and results of operations of the Company.

Date: May 9, 2016	/s/ DANIEL N. SWISHER, JR.
	Daniel N. Swisher, Jr.   Chief Executive Officer and President
Date: May 9, 2016	/s/ ERIC H. BJERKHOLT
	Eric H. Bjerkholt
	Executive Vice President, Corporate   Development and Finance,   Chief Financial Officer and Corporate Secretary

This certification accompanies the Form 10-Q to which it relates, is not deemed filed with the Securities and Exchange Commission and is not to be incorporated by reference into any filing of Sunesis Pharmaceuticals, Inc. under the Securities Act of 1933, as amended, or the Securities Exchange Act of 1934, as amended (whether made before or after the date of the Form 10-Q), irrespective of any general incorporation language contained in such filing.