Gene therapy products represent a novel and complex class of products. Ensuring product safety, identity, purity and potency following a manufacturing change extends not only to assessing the final formulated product but also all the components used during product manufacturing. CBER has implemented a stepwise approach to product characterization and compliance with cGMPs, which increases as the study moves from phase 1 toward phase 3 and licensing. It is important that product characterization be performed early in product development because without full product characterization it will be difficult to determine the impact of the manufacturing process on the product as well as the impact any manufacturing change will have on the product. To demonstrate product comparability a thorough understanding of the manufacturing process, including product characterization, is necessary, so that the impact of a manufacturing change can be accurately assessed.