TNFSF9, also known as 4-1BBL, is a member of the tumor necrosis factor (TNF) superfamily and is the ligand for the TNFR superfamily receptor, 4-1BB. 4-1BB plays an immunomodulatory role in T cells and NK cells. Human h4-1BB has been targeted with agonists in cancer immunotherapy. The structure of 4-1BBL has be revealed [, , ].