Leukemia and myelodysplasia arose in a gammaretroviral gene therapy trial for adenosine deaminase-deficient severe combined immune deficiency (ADA-SCID), representing a setback; however, this side effect led to the development of the self-inactivating (SIN) γ vector (Hacein-Bey-Abina et al., 2014). Here, ADA is linked to leukemia.