4-1BBL is a member of the tumor necrosis factor (TNF) receptor family. This receptor contributes to the clonal expansion, survival, and development of T cells. In addition, 4-1BBL expression is found on dendritic cells, follicular dendritic cells, natural killer cells, granulocytes and cells of blood vessel walls at sites of inflammation. CD137 has been shown to interact with TRAF2. The human 4-1BBL gene codes for a 254 amino acid type II transmembrane containing a 28 amino acid cytoplasmic domain, a 21 amino acid transmembrane domain, and a 205 amino acid extracellular domain (ECD). The human 4-1BBL ECD shares 32 % and 35 % a.a. identity with Mouse and rat ECD.