IL7R and severe combined immunodeficiency: The critical role of the IL-7 pathway for human T cell homeostasis is illustrated by the fact that absence of a functioning IL-7Rα leads to severe combined immunodeficiency with a T-B + NK + phenotype (14), while somatic gain-of-function mutations in IL-7Rα may cause T- as well as B-cell acute lymphoblastic leukemia (15, 16).