4-1BBL (aka CD137L or TNFSF9) is a cytokine that belongs to the tumor necrosis factor (TNF) ligand family. This transmembrane cytokine is a bidirectional signal transducer that acts as a ligand for TNFRSF9/4-1BB, which is a costimulatory receptor molecule in T lymphocytes. 4-1BBL and its receptor are involved in the antigen presentation process and in the generation of cytotoxic T cells; they are also involved in promoting T lymphocyte proliferation. 4-1BBL has been shown to be required for the optimal CD8 responses in CD8 T cells. 4-1BBL is expressed in carcinoma cell lines, and is thought to be involved in T cell-tumor cell interaction. [taken from NCBI Entrez Gene (Gene ID: 8744)].