PDCD1 and cancer: Therapeutic blockade of PD-1, PD-ligand1 (PD-L1) and CTLA-4 by monoclonal antibodies has demonstrated clinical efficacy in cancer by enhancing tumour-specific T-cell function,10,11 but monoclonal antibodies to multiple other immune checkpoints including LAG-3, TIM-3 and TIGIT are currently under active clinical development.12