The potential pathogenicity of replication competent retrovirus (RCR) requires vigilant testing to exclude the presence of RCR in vector-based human gene therapy products (Ref. 1). We, the FDA, are providing you, sponsors of retroviral vector-based human gene therapy products,recommendations regarding the testing for RCR during the manufacture of retroviral vector based gene therapy products, and during follow-up monitoring of patients who have received retroviral vector-based gene therapy products. Recommendations include the identification and amount of material to be tested as well as general testing methods. In addition, recommendations are provided for monitoring patients for evidence of retroviral infection after administration of retroviral vector-based gene therapy products.

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