Self-inactivating lentiviral vectors with an enhanced safety profile have been used in numerous clinical trials for ex vivo HSC gene addition/editing, providing remarkable therapeutic benefits in patients with severe inherited blood disorders such as the Wiskott–Aldrich syndrome (WAS) [108,109], X-linked and ADA severe combined immunodeficiency (SCID-X1, ADA-SCID) [110,111], β-hemoglobinopathies [112] and neurodegenerative storage diseases (adrenoleukodystrophy and metachromatic leukodystrophy) [113,114]. The gene discussed is ADA; the disease is severe combined immunodeficiency.