Apoptosis, or programmed cell death, occurs during normal cellular differentiation and development of multicellular organisms. Apoptosis is induced by certain cytokines including TNF and Fas ligand of the TNF family through their death domain (DD)-containing receptors, TNFR1 and Fas. The death signals are transduced by a group of DD-containing adapter molecules. A novel cell death adapter was recently identified by two independent groups and designated RAIDD (RIP-associated ICH-1/CED-3-homologous protein with DD) and CRADD (caspase and RIP adapter with DD)1, RAIDD contains a DD and a CARD (for caspase recruitment domain) which interact with RIP and caspase, respectively, to transduce death signals1, 3. RAIDD is constitutively expressed in many tissues and mediates apoptosis caused by Fas and TNFR-1.