Cluster of differentiation 27 (CD27) is a member of the tumor necrosis factor receptor superfamily and plays a key role in T-cell activation by providing a costimulatory signal. Bound to its natural ligand CD70, CD27 signaling enhances T-cell proliferation and differentiation to effector and memory T cells and therefore has potential as an immune modulatory target in cancer treatment [PMID: 32152062].