4-1BB ligand (4-1BBL; also CD137L) is a 32kD type II transmembrane protein that belongs to TNF superfamily (TNFSF) of molecules. 1-4 The human 4-1BBL cDNA encodes a 254aa protein that contains a 25aa N-terminal cytoplasmic domain, a 23aa transmembrane segment, and a 206aa C-terminal extracellular region. 5 The extracellular domain (ECD) of 4-1BBL has a jelly-roll, beta-sandwich tertiary structure that is similar to other TNFSF members. There is only one cysteine in the human ECD, and no potential N-linked glycosylation sites. The potential exists, however, for O-linked glycosylation. The human 4-1BBL ECD shares 32% and 35% aa identity with mouse and rat ECD, respectively. In the cytoplasmic domain, human 4-1BBL is 55aa shorter than the equivalent region in rodents. 4-1BBL is expressed by activated B cells, macrophages, dendritic cells, activated T cells, neurons, and astrocytes. 2, 3, 6 A 26kD soluble form of 4-1BBL is known to occur in human. Although it is presumably generated by MMP activity, its amino acid size is currently unreported. 4 The soluble form is bioactive. Human 4-1BBL signals through both CD137/4-1BB and itself. Its cytoplasmic tail participates in reverse signaling that induces apoptosis in T cells and cytokkine secretion (IL-6; TNF-alpha) by monocytes. 7, 8 4-1BBL binding to CD137/4-1BB produces a number of effects. It seems to play a key role in the T cell recall response. It maintains T cell numbers at the end of a primary response, and induces CD4 + and CD8 + T cells to proliferate and secrete cytokines such as IL-2 and IFN-γ in CD4 + cells, and IFN-γ in CD84-1BB Ligand/TNFSF9.