Gene therapy (GT) with autologous haematopoietic stem/progenitor cells (HSPC) is a promising treatment for primary immunodeficiency, such as adenosine deaminase-severe combined immunodeficiency (ADA-SCID) [1–4] and Wiskott–Aldrich syndrome (WAS) [5, 6], and inherited metabolic disorders such as metachromatic leukodystrophy (MLD) [7, 8]. The gene discussed is ADA; the disease is inborn error of immunity.