ADA and T-B+ severe combined immunodeficiency due to gamma chain deficiency: In the 1980s, the concept formed of using a virus vector for gene transfer into mammalian cells (Williams et al., 1984), and, in 1990, the first approved gene therapy trial took place with viral vector-mediated transfer of the gene encoding the enzyme adenosine deaminase (ADA) in a 4-year-old patient suffering from chromosome X-linked severe combined immunodeficiency (SCID-X1) due to ADA deficiency (Blaese et al., 1995).