After pioneering studies with a cytotoxic T-lymphocyte associated antigen 4 (CTLA-4; cluster of differentiation (CD) 152) blockade that resulted in the Food and Drug Administration (FDA)’s approval of ipilimumab for the treatment of advanced melanoma in 2011, programmed cell death protein 1 (PD-1; CD279)/PD-1 ligand 1 (PD-L1) signaling is in the focus of the current research on, and the development of, anti-tumor therapy in this field. The gene discussed is PDCD1; the disease is neoplasm.