TNFSF9, also termed 4-1BBL and CD137L, is a type II transmembrane glycoprotein that belongs to the TNF ligand superfamily. This protein is mainly expressed on antigen presenting cells (APC), particularly B lymphocytes. The interaction of TNFSF9 and its receptor, CD137, which is mainly expressed on T cells, leads to activation of cytotoxic T cells. In addition, TNFSF9-CD137 interaction can reactivate anergic T lymphocytes as well as induce T cell proliferation. This molecule plays a role in optimal CD8 T cell response. Interestingly, it appears that TNFSF9 is able to act as a costimulatory molecule without the engagement of other costimulatory molecules such as CD28. TNFSF9 signaling on T cells has also been shown to limit T cell activation. CD137L is upregulated in peripheral CD14+ monocytes of chronic hepatitis B patients and is associated with liver cirrhosis. Human TNFSF9 shares 36% of its amino acid sequence with its murine counterpart. Mice deficient in TNFSF9 are predisposed to develop B-cell lymphomas.