The US Food and Drug Administration (FDA) has issued a draft guidance for testing the potency of cellular and gene therapy (CGT) products. The guidance clarifies the potency information needed to support an Investigational New Drug Application or a Biologics License Application for products reviewed by the FDA's Office of Cellular, Tissue and Gene Therapies. There was “an urgent need for this document,” says the FDA's Denise Gavin, as many CGTs have been held up going into phase 3 trials owing to difficulties establishing appropriate potency tests. The document outlines three potency measurement categories—biological assays, nonbiological assays and matrix assays—used individually or in combination. The guidance does not cover the selection of an assay, however, because the inherent variability of CGT products requires appropriate potency measurements for an individual product. Companies are responsible for devising their own potency assays, and the FDA evaluates their adequacy on a case-by-case basis. The guidance recommends that companies start determining which product attributes are related to potency even at the preclinical stage. “The main thing is don't delay your product characterization,” says Gavin. He encourages companies to discuss their plans with the FDA but acknowledges that potency tests “may change significantly” as products are developed. The draft is open for comment until 7 January 2009.