4‐1BBL, a member of the TNF superfamily, is expressed in B cells, dendritic cells, activated T cells and macrophages. 4‐1BBL binds to its receptor 4‐1BB, and provides a co‐stimulatory signal for T cell activation and expansion. The human 4‐1BBL gene codes for a 254 amino acid type II transmembrane protein containing a 28 amino acid cytoplasmic domain, a 21 amino acid transmembrane protein domain, and a 205 amino acid extracellular domain. The soluble form of 4‐1BBL contains the TNF‐like portion of the extracellular domain of 4‐1BBL. Recombinant Human 4‐1BB Ligand is a soluble 19.5 kDa protein consisting of 185 amino acid residues.