FAF1 (Fas-associated protein factor 1) specifically interacts with the cytoplasmic domain of wild type, but not mutated, Fas in both yeast and mammalian cells. When transiently expressed in T cells, FAF1 potentiates Fas-induced apoptosis. In contrast to TRADD, FADD, and RIP, FAF1 lacks a DDH (Death domain homologous region) and cannot induce apoptosis independent of Fas activation.