Tumor necrosis factor ligand superfamily member 1 (TNFSF1), also known as TNF-related apoptosis-inducing ligand (TRAIL), Apo-2 ligand, and CD253, is a cytokine that belongs to the tumor necrosis factor (TNF) ligand family. TNFSF1 / Apo-2L / CD253 functions as a ligand that induces the process of cell death called apoptosis. TNFSF1 / TRAIL shows homology to other members of the tumor necrosis factor superfamily. As one member of the cluster of differentiation system, TNFSF1 / CD253 is commonly used as cell markers in immunophynotyping. Different kinds of cells in the immune system can be identified through the surface CD molecules which associating with the immune function of the cell. There are more than 32 CD unique clusters and subclusters have been identified. Some of the CD molecules serve as receptors or ligands important to the cell through initiating a signal cascade which then alter the behavior of the cell. Some CD proteins do not take part in cell signal process but have other functions such as cell adhesion TNFSF1 / Apo-2L / CD253 / TRAIL binds to several members of TNF receptor superfamily including TNFRSF1A / TRAILR1, TNFRSF1B / TRAILR2, TNFRSF1C / TRAILR3, TNFRSF1D / TRAILR4, and possibly also to TNFRSF11B/OPG. The activity of TNFSF1 / TRAIL may be modulated by binding to the decoy receptors TNFRSF1C / TRAILR3, TNFRSF1D/TRAILR4, and TNFRSF11B/OPG that cannot induce apoptosis. The binding of this protein to its receptors has been shown to trigger the activation of MAPK8 / JNK, caspase 8, and caspase 3. Alternatively spliced transcript variants encoding different isoforms have been found for this gene.