Although IL-2 was originally described as a T cell stimulatory factor (Morgan et al., 1976), mice deficient in IL-2 or the IL-2 receptor were found to develop severe autoimmunity (Sadlack et al., 1995; Sadlack et al., 1993; Suzuki et al., 1995; Willerford et al., 1995). The gene discussed is IL2; the disease is Autoimmunity.