Most previous murine studies have used lpr or gld mice, which have defects in Fas and the Fas ligand, respectively (4, 6–8), leading to defective apoptosis within lymphoid tissue, severely impaired T cell differentiation, and massive hepatosplenomegaly and lymphadenopathy in mice, and the lymphoma-like Canale-Smith syndrome in humans (28). This evidence concerns the gene FASLG and autoimmune lymphoproliferative syndrome.