The importance of the CD95/CD95L-system for lymphocyte homeostasis became apparent from the initial observation that naturally occurring mice which developed massive lymphadenopathy and suffered from lymphoproliferative syndromes carried the causative mutations in the genes encoding either for CD95 (lpr = lymphoproliferation) or for CD95L (gld = generalized lymphoproliferative disease). The gene discussed is FASLG; the disease is lymphoproliferative syndrome.