35 U.S. Code § 323 - Preliminary response to petition
If a post-grant review petition is filed under section 321, the patent owner shall have the right to file a preliminary response to the petition, within a time period set by the Director, that sets forth reasons why no post-grant review should be instituted based upon the failure of the petition to meet any requirement of this chapter.
Section effective upon the expiration of the 1-year period beginning Sept. 16, 2011, and applicable only to patents described in section 3(n)(1) of Pub. L. 112–29 (35 U.S.C. 100 note), with certain exceptions and limitations, see section 6(f)(2), (3) of Pub. L. 112–29, set out as a note under section 321 of this title.