37 C.F.R. § 42.107 : Patents, Trademarks, and Copyrights — UNITED STATES PATENT AND TRADEMARK OFFICE, DEPARTMENT OF COMMERCE — GENERAL PATENTS — TRIAL PRACTICE BEFORE THE PATENT TRIAL AND APPEAL BOARD — INTER PARTES REVIEW —GENERAL — Preliminary response to petition

(a) The patent owner may file a preliminary response to the petition. The response is limited to setting forth the reasons why no inter partes review should be instituted under 35 U.S.C. 314 and can include supporting evidence. The preliminary response is subject to the word count under §42.24.
(b) Due date. The preliminary response must be filed no later than three months after the date of a notice indicating that the request to institute an inter partes review has been granted a filing date. A patent owner may expedite the proceeding by filing an election to waive the patent owner preliminary response.
(c) [Reserved]
(d) No amendment. The preliminary response shall not include any amendment.
(e) Disclaim Patent Claims. The patent owner may file a statutory disclaimer under 35 U.S.C. 253(a) in compliance with §1.321(a) of this chapter, disclaiming one or more claims in the patent. No inter partes review will be instituted based on disclaimed claims.

37 C.F.R. §42.107

77 FR 48727, 8/14/2012, as amended at 81 FR 18766, 4/1/2016