Tumor necrosis factor ligand superfamily member 9(4-1BBL) is single-pass type II membrane protein which is a member of the the tumor necrosis factor family. 4-1BBL is a 254 amino acids cytokine that is expressed in brain, placenta, lung, skeletal muscle and kidney. TNFSF9 has been shown to reactivate anergic T lymphocytes in addition to promoting T lymphocyte proliferation. This cytokine may have a role in activation-induced cell death (AICD) and cognate interactions between T-cells and B-cells/macrophages. It has also been shown to be required for the optimal CD8 responses in CD8 T cells, and is thought to be involved in T cell-tumor cell interaction.