4-1BBL is found on APCs (antigen presenting cells) and binds to 4-1BB. Therapies utilizing the 4-1BB:4-1BBL signaling pathway have been shown to have antitumor effects in a number of model systems. It is a membrane bound member of the TNF superfamily that is expressed on activated B-cells; macrophage; and dendritic cells. The ligand is specific for the 4-1BB receptor and may play a role in inducing the proliferation of activated peripheral blood T-cells.