Lentivirus-based hematopoietic stem cell gene therapy has been used to treat X-linked adrenoleukodystrophy (X-ALD) [75], the lysosomal storage disease metachromatic leukodystrophy (MLD) [76, 77], β-hemoglobinopathies like β-thallassemia [78–80] and sickle-cell disease [81], and not least primary immunodeficiencies including ADA-SCID [82], SCID-X1 [83], Wiskott-Aldrich Syndrome (WAS) [84–86], and X-linked chronic granulomatous disease (X-CGD) [87]. The gene discussed is ADA; the disease is metachromatic leukodystrophy.