X-linked SCID (X-SCID) and adenosine deaminase-deficient severe combined immunodeficiency (ADA-SCID) were the first genetic diseases to be successfully treated by gene therapy [7–13] eventually resulting in Strimvelis as the first, and to date only, ex vivo gene therapy with market authorization for the treatment of ADA-SCID [14]. This evidence concerns the gene ADA and severe combined immunodeficiency.