Fas and tumor necrosis factor receptor 1 (TNFR1) are two prototype members in the death receptor family. A novel protein that associates with the intracellular domains of Fas and TNFR1 was recently identified and designated F1Aalpha and FEM1β. F1Aalpha/FEM1β is the homologue of C. elegans sex determining protein FEM-1. FEM-1/F1Aalpha is cleaved by CED-3 and caspase. FEM-1/F1Aalpha associates with CED-4 and its mammalian homologue Apaf-1. Overexpression of F1Aalpha induces apoptosis. F1Aalpha is therefore a novel member of the death receptor associated protein that mediates apoptosis. F1Aalpha is expressed in a variety of human and mouse tissues.