ADA and severe combined immunodeficiency: The widespread success of first-generation gammaretrovirus-based vectors for human gene-therapy resulted from their use in the correction of primary immunodeficiencies, such as X-linked severe combined immunodeficiency (SCID-X1), adenosine deaminase-SCID (ADA-SCID), Wiskott–Aldrich syndrome (WAS) and X-linked chronic granulomatous disease (CGD) (reviewed in 154–157).