Although γc was initially discovered as the IL-2 receptor γ chain (IL-2Rγ) and identifying the genetic basis for XSCID resulted from studies of the IL-2R, the fact that the phenotype in XSCID is more severe than in IL-2 deficiency led to the prediction and then discovery that IL-2Rγ was in fact a shared receptor component5–8, and the term γc was proposed5. This evidence concerns the gene IL2 and T-B+ severe combined immunodeficiency due to gamma chain deficiency.