The management of metastatic dMMR/MSI-H CRC has recently been transformed by clinical data demonstrating remarkable clinical benefit of PD-1 inhibitors in this setting.13–16 Mechanistically, this is thought to relate to the high number of neoantigens in these tumours,13 and the reversal of the strong upregulation of immune checkpoints (eg, PD-1, PD-L1, cytotoxic T lymphocytes-associated protein-4 (CTLA-4), LAG-3 and IDO) which may overcome a phenomenon referred to as ‘adaptive immune resistance’. This evidence concerns the gene CTLA4 and neoplasm.