PDCD1 and cancer: T-cell checkpoint inhibitors, such as the anti-cytotoxic T-lymphocyte-associated protein 4 (CTLA4), monoclonal antibody, such as ipilimumab, and the anti-programmed cell death protein 1 (PD-1) monoclonal antibodies, including pembrolizumab and nivolumab, have improved patient prognosis in a number of cancer types [4].