The initial GT clinical trials for hematopoietic disorders were conducted in the 1990s and utilized long terminal repeat (LTR)-driven γ-retroviral vectors (γRVs) aimed at gene-based correction of functional defects underlying several primary immunodeficiencies, including X-linked severe combined immunodeficiency (SCID-X1),21, 22, 23, 24 adenosine deaminase (ADA)-SCID,25, 26, 27 chronic granulomatous disease (CGD),28,29 and Wiskott-Aldrich syndrome (WAS).30 This evidence concerns the gene ADA and T-B+ severe combined immunodeficiency due to gamma chain deficiency.