PDCD1 and neoplasm: The research of tumor immune checkpoint inhibitors mainly focuses on three molecules: cytotoxic T-lymphocyte-associated protein 4 (CTLA-4), programmed cell death protein 1 (PD-1) and its ligand PD-L1, such as camrelizumab, sintilimab, pembrolizumab, atezolizumab, tislelizumab, nivolumab, ipilimumab.