Ariad Pharmaceuticals, Inc. (Ariad) (plaintiff) obtained a patent for its discovery that reducing NF-kB activity could reduce the harms of certain diseases. Ariad identified a transcription factor called NF-kB that activates genes that help fight off infections. Ariad hypothesized that artificially reducing NF-kB could alleviate the symptoms of certain diseases, like aspirin can reduce fever symptoms. It filed for a patent describing several methods of reducing NF-kB activity in cells and hypothesizing three classes of molecules that might do this: specific inhibitors, dominantly interfering molecules, and decoy molecules. Eli Lilly & Co. (Eli Lilly) (defendant) manufactured two drugs that worked by reducing NF-kB activity. Ariad brought suit against Eli Lilly for infringement of the patent. The United States District Court for the District of Massachusetts (District Court) found patent infringement, but did not hold any of Ariad’s claims invalid. The District Court then denied Ariad’s motion for judgment as a matter of law. A panel of the United States Court of Appeals for the Federal Circuit reversed the denial and held Ariad’s claims invalid for want of written description. Ariad petitioned for a rehearing en banc, arguing that the first paragraph of 35 U.S.C. § 112 does not contain a written description requirement separate from the enablement requirement, and, alternatively, that if a separate written description is required, Ariad’s description was sufficient to meet this requirement.