Fas ligand (FasL or CD95L) is a cytokine that binds to TNFRSF6/FAS, a receptor that transduces the apoptoic signal into cells. FasL is involved in cytotoxic T-cell mediated apoptosis and in T-cell development. TNFRSF6/FAS-mediated apoptosis has a role in the induction of peripheral tolerance, in the antigen-stimulated suicide of mature T cells or both. Defects in FasL are the cause of autoimmune lymphoproliferative syndrome 1B (ALPS1B), a childhood syndrome involving hemolytic anemia and thrombocytopenia.