PDCD1 and cancer: Cancer cells can take advantage of signaling through the interaction of immune factors, such as cytotoxic T-lymphocyte-associated protein 4 (CTLA-4), which binds to the B7 ligand, and programmed cell death protein 1 (PD-1), which binds to programmed death ligand-1 (PD-L1) to promote tumor development while evading attacks from the immune system.