What protection does 35 U.S.C. 121 provide to patent holders regarding improper joinder?

This page is an FAQ based on guidance from the Manual of Patent Examining Procedure. It is provided as guidance, with links to the ground truth sources. This is information only: it is not legal advice.

35 U.S.C. 121 provides significant protection to patent holders regarding improper joinder of inventions. According to MPEP 805, the statute states:

“the validity of a patent shall not be questioned for failure of the Director to require the application to be restricted to one invention.”

This means that patent holders are protected from challenges to their patent’s validity based on the inclusion of multiple inventions in a single patent. Even if the USPTO did not require the applicant to separate different inventions into distinct applications, this cannot be used as a basis for invalidating the resulting patent.