PDCD1 and neoplasm: To avoid immunosurveillance, tumor cells always upregulate the expression of immune checkpoint-related genes, such as programmed cell death protein-1 (PD-1) and cytotoxic T lymphocyte antigen 4 (CTLA4), during tumor development, which will cause T cell anergy or apoptosis (Leach et al., 1996; Iwai et al., 2002; Chan et al., 2019).