Binding or cross linking of the Fas antigen (also known as APO-1 and CD95) is known to elicit apoptosis in susceptible cells. Fas is a member of a family of cell surface receptors which includes tumor necrosis factor receptors (TNF-R, and TNF-R2) and nerve growth factor receptors (NGF-R), CD40, OX40, CD30, CD27, and 4-1BB. Several members of this family have been shown to regulate or induce cell death (TNF-R1 and TNF-R2). A 74 kDa member of this family protein named RIP (Receptor Interacting Protein) contains an N-terminal region with homology to protein kinases and a C-terminal region containing a cytoplasmic "death domain" present in both Fas and TNF-R1. Both Fas and RIP have been shown to require this death domain to induce apoptosis and overexpression of RIP has been shown to induce cell death in transfected cells.