Inclusion Criteria  Subjects must have received gRV-GT as part of a clinical trial, expanded access program, as an approved product (Strimvelis) or under a post-approval hospital exemption, and must have at least one retained biological sample that meets the following eligibility criteria:  Peripheral blood, bone marrow, or DNA extracted from either source. Taken at least 6 months after gRV-GT. Stored at -20oC or below since the time of sampling. Likely to provide at least 1.5 μg of DNA (following extraction by the central laboratory).  There is no restriction on the number of samples from a single subject, or on the time that has elapsed since sampling.  Exclusion Criteria N/A