PDCD1 and cancer: The Programmed Cell Death 1 (PD-1 or CD279) protein is a T-cell co-inhibitory receptor, which upon binding of its ligand PD-L1 (CD274) expressed by tumor cells, inhibits cytokine production and cytotoxic activity of PD-1+ tumor infiltrating T-lymphocytes, facilitating tumor progression (escape phase of cancer immunoediting).