TNF-like weak inducer of apoptosis (TWEAK) is a type II transmembrane TNF superfamily member with strong homology to TNF in its extracellular portion, though less potent in inducing apoptosis. It has been clustered as CD255. TWEAK transcript is expressed broadly in many adult and fetal tissues; however, the staining of human peripheral blood mononuclear cells with TWEAK-specific antibodies shows a more restricted pattern. TWEAK is expressed as membrane-bound and secreted forms. Interaction of TWEAK with its receptors (DR3, Fn14) promotes secretion of IL-8, activation of NF-kappaB, proliferation of endothelial cells, and apoptosis in a number of human cell lines.