PDCD1 and neoplasm: All four of these ICISs exert their activity by blocking either PD-1 (nivolumab and pembrolizumab) or PD-L1 (atezolizumab and durvalumab), interfering with and partially preventing tumor cells’ immune-escape, thus enhancing patients’ immune surveillance and T-cell-mediated responses to cancer cells [22,23,24].