PDCD1 and neoplasm: Recently, the PD-1/PD-L1 pathway regulates tumor microenvironment through the induction and maintenance of immune tolerance, and PD-1 and PD-L1 have been proved to be promising targets for the treatment of a large number of tumor types, such as hepatocellular carcinoma (HCC), pancreatic cancer, gastric neuroendocrine carcinomas, thymic carcinoma, non-small-cell lung cancer (NSCLC) (16–20).