Autoimmune Lymphoproliferative Syndrome (ALPS) is a recently recognized disease in which a genetic defect in programmed cell death, or apoptosis, leads to breakdown of lymphocyte homeostasis and normal immunologic tolerance. Some authors have referred to ALPS as Canale-Smith syndrome or lymphoproliferative syndrome with autoimmunity. Patients with ALPS have chronic enlargement of the spleen and lymph nodes, various manifestations of autoimmunity, and elevation of a normally rare population of "double negative T cells" (DNTs), T lymphocytes bearing alpha beta T cell receptors and expressing neither cluster differentiation (CD)4 nor CD8 surface antigens. When lymphocytes from patients with ALPS are cultured in vitro, they are resistant to apoptosis as compared to cells from healthy controls. Most patients with ALPS have mutations in a gene now named TNFRSF6 (tumor necrosis factor receptor gene superfamily member 6). This gene, previously known as apoptosis antigen 1 (APT1), encodes the cell surface receptor for the major apoptosis pathway in mature lymphocytes; this receptor has also had many names, including Fas (to be used here), CD95, and APO-1. ALPS is subdivided into: 1) Type Ia, ALPS with mutant Fas; 2) Type Ib, lymphadenopathy and mutation in the ligand for Fas in one patient with systemic lupus erythematosus; 3) Type II, ALPS with mutant caspase 10; and 4) Type III, ALPS as yet without any defined genetic cause.