The patent licensing occurs when the licensor (patent holder) grants exploitation rights over a patent to a licensee for royalty or a lump sum payment. A licensee can challenge the validity of a licensed patent to avoid paying royalties, though there are some limitations, such as not being able to challenge after failing to pay royalties. The Supreme Court's MedImmune v. Genentech decision found that licensees can challenge licensed patents even while continuing to pay royalties, increasing licensees' freedom to challenge validity. Licensors may respond by negotiating higher royalties, up-front payments, or termination clauses.