PDCD1 and central nervous system cancer: In recent years, immune checkpoint inhibitors, such as programmed cell death protein 1 (PD-1) [2], cytotoxic T-lymphocyte-associated antigen 4 (CTLA-4) [3], and indoleamine 2,3-dioxygenase (IDO) [4, 5], have been successfully used in cancer therapy, thus attracting increased interest in the development of immune checkpoint inhibitors for glioma.