Tumor necrosis factor (TNF) and lymphotoxin-alpha (LT-(alpha), also known as TNF?(beta)) are members of a family of secreted and cell surface cytokines that participate in the regulation of immune and inflammatory responses. LT-(beta) (lymphotaxin-beta or tumor necrosis factor C) is a type II membrane protein with significant homology to TNF, LT-(alpha), and the ligand for the CD40 receptor. LT-(alpha) is present on the surface of activated T, B, and LAK cells as a complex with the 33 kda glycoprotein, LT-(beta). LT-(beta), also expressed by active lymphocytes, forms a heterotrimer with LT-a on the cell surface and anchors LT-(alpha) to the cell surface. A TNF receptor-related protein, the LT-(beta) receptor (also known as TNFC receptor), is the human receptor for the LT-(alpha)/LT-(beta) heterotrimer. There are two LT-(beta) isoforms expressed in human lymphoid cell lines and non-Hodgkin's lymphomas. The gene which encodes LT-(beta) maps to the major histocompatibility complex region on human chromosome 6p21.3.