The USPTO set a time limit for requesting ex parte reexamination to align with the period of enforceability of a patent. As explained in MPEP 2211, "the Office considered that Congress could not have intended expending Office resources on deciding patent validity questions in patents which cannot be enforced." This rationale was supported in the case of Patlex Corp. v. Mossinghoff, 758 F.2d 594, 225 USPQ 243, 249 (Fed. Cir. 1985). The time limit ensures that USPTO resources are focused on patents that can still be enforced. To learn more: ex parte reexamination patent enforceability USPTO resources patent validity