PDCD1 and cancer: Up to now, the US Food and Drug Administration (FDA) has approved several ICIs for cancer treatment, including one CTLA-4 inhibitor (ipilimumab), three PD-1 inhibitors (nivolumab, pembrolizumab, cemiplimab), and three PD-L1 inhibitors (atezolizumab, avelumab, durvalumab) [4].