In recent years the Federal Circuit, in reviewing both litigation and prosecution decisions, has revived the doctrine of written description under 35 U.S.C. § 112. The court has applied the doctrine to biotechnology patents—specifically those claiming genuses of antibodies, DNA sequences or other biologic molecules (i.e., molecules made by processes occurring in living cells). These claims are frequently the target of written description attacks by both the U.S. Patent and Trademark Office and by private litigants. In this article, we review several recent Federal Circuit decisions on the written description requirement and some ways that biotech IP stakeholders can draft patents that stand up better to the new § 112 law.