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This guidance provides FDA’s current thinking on determining sameness of human gene therapy products1 under FDA’s orphan drug regulations for the purpose of orphan-drug designation and orphan-drug exclusivity. This guidance is intended to assist stakeholders, including industry and academic sponsors who seek orphan-drug designation and orphan-drug exclusivity, in the development of gene therapies for rare diseases. This guidance focuses specifically on factors that FDA generally intends to consider when determining sameness for gene therapy products and does not address sameness determinations for other types of products.