TNFSF9, also known as 4-1BBL and CD137L, is a type II transmembrane glycoprotein that belongs to the TNF superfamily. This protein is mainly expressed on antigen presenting cells (APC), and on B lymphocytes particularly. The interaction between TNFSF9 and its receptor, CD137, which is mainly expressed on T cells, leads to the activation of cytotoxic T cells. In addition, TNFSF9-CD137 interaction can reactivate anergic T lymphocytes and 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 of TNFSF9 are susceptible to develop B-cell lymphomas.