PDCD1 and brain neoplasm: The crucial checkpoints in pediatric brain tumors are cytotoxic T lymphocyte antigen-4 (CTLA-4), programmed cell death protein-1 (PD-1) and programmed death-ligand 1 (PD-L1), OX-2 membrane glycoprotein (CD200), and indoleamine 2,3-dioxygenase (IDO).