The TNF ligand family member termed TRAIL has been shown to induce apo
ptosis in a wide variety of transformed cell lines. The normal functio
ns of this cytokine in vivo remain, however, relatively unknown. The c
omplexity of this biological system has now increased unexpectedly wit
h the identification of four distinct receptors for TRAIL, two of whic
h have cytoplasmic death domains. This review will describe the known
biological effects of TRAIL, as well as the structure and possible fun
ctions of its recently identified receptors.