CD8A and severe combined immunodeficiency: Loss-of-function mutations or lack of ZAP-70 expression leads to a rare form of severe combined immunodeficiency (SCID) in both humans and mice [10], as in its absence the development of T cells is arrested in the double positive stage (CD4+CD8+), resulting in virtually no mature αβ T cells in the peripheral lymphoid organs in mice, and only CD4+ αβ T cells in humans, with impaired activation capacity through the TcR [11].