PDCD1 and neoplasm: PD-1 and its ligand PD-L1 represent an inhibitory immune checkpoint at the tissue level, wherein PD-L1 expressed on tumour tissue binds PD-1 on cytotoxic T cells and leads to T-cell anergy.94,95 Targeting this checkpoint has proven successful in other tumour types96, , , , , –102 and its activity in GBM is currently being explored.