The origin of the dispute was a lengthy interference proceeding between Genentech and Celltech which led to the issuance of a new patent in 2001, 18 years after the original filing. This effectively granted Genentech a patent term of 29 years. MedImmune was a licensee of the later Cabilly patent, but argued that the term had been improperly extended and that it need not continue to pay royalties past the original expiry date in March 2006. The case was decided in favor of MedImmune, and the United States Patent and Trademark Office (USPTO) declared the patent invalid. However, Genentech has appealed the USPTO's ruling and the patent will remain valid and enforceable until the appeal is concluded.