CD8A and neoplasm: A member of the tumor necrosis factor (TNF) receptor superfamily that includes the FAS receptor (apoptosis antigen), CD40 (T cell costimulatory receptor), CD27 (TNF receptor), and CD30 (tumor marker), and 4-1BB (CD137) is a Type II transmembrane glycoprotein [118] that is inducibly expressed on primed CD4+ and CD8+ T cells [119], activated NK cells, DCs, and neutrophils [120] and acts as a T cell costimulatory molecule when bound to the 4-1BB ligand (4-1BBL) found on activated macrophages, B cells, and DCs [121, 122].