Cytokines play a major role in lymphoid development. Defects of the common gamma chain (gamma(c)) or of the JAK3 protein in humans have been shown to result in a severe combined immune deficiency (SCID), with a profound defect in T and natural killer (NK)-cell development, whereas B-cell generation is apparently unaffected (T-B+NK-SCID). While extensive molecular and biochemical analysis of these patients has been instrumental in understanding better the biological properties of the gamma(c) and JAK3 protein, an unexpected phenotypic heterogeneity of gamma(c) and JAK3 deficiency has emerged, indicating the need for appropriate and extensive investigations even in patients with atypical presentations. At the same time, characterization of the defects has been instrumental in the development of novel therapeutic approaches, from in utero hematopoietic stem cell transplantation to gene therapy.