PDCD1 and cancer: Thus, through the blockade of CLTLA-4 and/or the PD-1/PD-L1 system, ICIs have provided a novel mechanism for treating cancer, that is reactivation of the immune-mediated tumour killing [13,14], which is associated, from a clinical point of view, with durable responses and a favourable safety profile in patients for whom other cancer therapies have failed [15].