Therefore, the silencing of genes encoding molecules involved in this signalling axis, including programmed cell death 1 (PD1) and cytotoxic T-lymphocyte antigen 4 (CTLA-4), which are inhibitory receptors related to T cell exhaustion and the immune escape of tumour cells, are being studied with the aim of increasing cytotoxic activity [24, 35, 38, 48]. This evidence concerns the gene PDCD1 and neoplasm.