Tumor necrosis factor ligand superfamily member 9, also known as 4-1BBL, is a member of the the tumor necrosis factor family. Mouse 4-1BBL cDNA encodes a 309 amino acid residues (aa) protein with an 82 aa N-terminal cytoplasmic domain, a 21 aa transmembrane domain and a 206 aa C-terminal extracellular domain. The extracellular domain of 4-1BBL has a tertiary structure similar to that of other TNFSF members, but shares only low aa sequence homology (14-16%). 4-1BBL is predominantly expressed on activated antigen presenting cells (APCs) such as B cells, macrophages and dendritic cells (DCs). It is also expressed on most T and B lymphoma cell lines. TNFSF9 has been shown to reactivate anergic T lymphocytes in addition to promoting T lymphocyte proliferation. This cytokine 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.