Historically, ADA deficiency was the first SCID condition for which a genetic and molecular cause was identified and eventually transplantation of ADA-SCID patients with γ-retrovirus mediated gene-corrected autologous HSCs (from bone marrow or umbilical cord blood) in the early 1990s constituted the first attempts of treating a PID with gene therapy. This evidence concerns the gene ADA and severe combined immunodeficiency.