PDCD1 and cancer: In the past decade, researches suggested that tumor cells’ immunologic escape of and aberrant human immune surveillance play essential roles in the carcinogenesis, progression, and metastasis of cancers (4), and studies focused on anticancer immune responses have achieved marked success of many malignant tumors in preclinical and clinical trials (5–9), the PD-1 (programmed cell death protein 1) and PD-L1 (programmed cell death-ligand 1) axis has been identified as one of the most encouraging findings in cancer immunotherapy (10).