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 in the TNF family through their death domain containing receptors, TNFR1 and Fas. A novel member in the TNF family was recently identified and designated TRAIL (for TNF-related apoptosis-inducing ligand) and Apo-2L (for Apo-2 ligand)1,TRAIL is a type II membrane protein and expressed in a variety of human tissues. Two novel death domain containing receptors DR4 and DR5 have been identified as the receptor for TRAIL3-6. Like TNF and Fas ligand, TRAIL induces apoptosis and NF-kappaB activation in many tissues and cells.