PDCD1 and cancer: Immune checkpoint proteins (ICPs) are pivotal factors in the regulation of immune responses and there are some immune checkpoint inhibitors, e.g., against PD-1/PD-L1 and cytotoxic T lymphocyte antigen 4 (CTLA-4), which showed promising therapeutic results, and their use was approved, for some cancer treatments, by the United States’ Food and Drug Administration (FDA) [85].