Gene therapy is an emerging technology that has demonstrated clinical efficacy and biological effect in treating diseases such as severe combined immune deficiencies (SCID-X1, ADA-SCID) [13,14] and chronic granulomatous disease (CGD) [15], and our own HIV study has demonstrated safety, persistence of gene-containing cells and a biological effect as detailed below [16]. This evidence concerns the gene ADA and chronic granulomatous disease.