TL1A, also known as TNFSF15, is a cytokine that belongs to the tumor necrosis factor (TNF) ligand family. It is specifically expressed in endothelial cells. TL1A also can be detected in monocytes, placenta, lung, liver, kidney, skeletal muscle, pancreas, spleen, prostate, small intestine and colon. TL1A is a ligand for receptor TNFRSF25 and decoy receptor TNFRSF21/DR6. It mediates activation of NF-kappa-B. It also inhibits vascular endothelial growth and angiogenesis (in vitro). TL1A promotes activation of caspases and apoptosis. It is also found to inhibit endothelial cell proliferation, and thus may function as an angiogenesis inhibitor.