PDCD1 and neoplasm: Upregulation of negative immune checkpoint molecule of programmed death ligand (PD-L1) in tumour cells, macrophages and dendritic cells under hypoxia, can initiate the interactions between PD-L1 and cell surface checkpoint receptor programmed cell death-1 (PD-1) expressed on effector T cells, resulting in the increased apoptosis of cytotoxic T lymphocytes (CTLs) and subsequent downregulation of T-cell antitumour reactivity.