These pioneer clinical protocols made their entry into the clinical arena inthe early 1990s and focused on patients affected with adenosine deaminase(ADA)-deficient severe combined immunodeficiency (SCID) who derived limited benefitfrom the genetic correction of either their peripheral blood lymphocytes or CD34+hematopoietic progenitors 4– 7. This evidence concerns the gene ADA and severe combined immunodeficiency.