▪ Abstract   Mutations in nine different genes have been found to cause the human severe combined immunodeficiency syndrome. The products of three of the genes—IL-2RG, Jak3, and IL-7Rα—are components of cytokine receptors, and the products of three more—RAG1, RAG2, and Artemis—are essential for effecting antigen receptor gene rearrangement. Additionally, a deficiency of CD3δ, a component of the T-cell antigen receptor, results in a near absence of circulating mature CD3+ T cells and a complete lack of γ/δ T cells. Adenosine deaminase deficiency results in toxic accumulations of metabolites that cause T cell apoptosis. Finally, a deficiency of CD45, a critical regulator of signaling thresholds in immune cells, also causes SCID. Approaches to immune reconstitution have included bone marrow transplantation and gene therapy. Bone marrow transplantation, both HLA identical unfractionated and T cell–depleted HLA haploidentical, has been very successful in effecting immune reconstitution if done in the first 3.5 months of life and without pretransplant chemotherapy. Gene therapy was highly successful in nine infants with X-linked SCID, but the trials have been placed on hold due to the development of a leukemic process in two of the children because of insertional oncogenesis.