X-linked severe combined immunodeficiency is characterized by severe and persistent infections from early life resulting from profound impairment of both cellular and humoral immune function. XSCID is characterized by an absence or diminished number of T cells and histologic evidence of hypoplastic and abnormal differention of the thymic epithelium. The discovery that this disease results from the mutations of the IL-2R gamma chain was surprising since IL-2-deficient mice and human SCID patients had milder phenotypes. This led to the speculation that IL-2R gamma would prove to be a common gamma chain, gamma c, which would play important roles in other cytokine receptors in addition to the IL-2 receptor. There is now compelling evidence to support a role in at least two other cytokine receptors, namely the IL-4 and IL-7 receptors. Thus, with inactivation of gamma c, multiple cytokine systems are simultaneously affected, resulting in the profoundly impaired phenotype of XSCID. It is possible and even likely that gamma c will be found to be a functional component of additional receptors as well. These findings have resulted in a significant improvement in our understanding of the pathophysiologic development of the defects in XSCID and also have important ramifications for prenatal and postnatal diagnosis, carrier female identification, and gene therapy for XSCID.