PDCD1 and neoplasm: Exploitation of immune checkpoint pathways is a major mechanism by which tumors escape immune surveillance, and immune checkpoint blockade is the basis for the clinical anti-tumor activity of most of the currently approved immuno-oncology agents targeting CTLA-4 (ipilumimab) and programmed cell death protein 1 (PD-1) (nivolumab, pembrolizumab,) or PD-1 ligand 1 (PD-L1) (atezolizumab, durvalumab, and avelumab) [1].