A recent study showed that the immune microenvironment of MSI-high (MSI-H) colorectal cancers was not only highly infiltrated with activated CD8+ cytotoxic T cells, but also showed upregulation of multiple immune checkpoints, including PD-1 (Programmed Death-1), PDL-1 (Programmed Death ligand-1), CTLA-4 (Cytotxic T cell antigen -4), LAG-3 (lymphocyte-activation gene 3), TIM3 (T-cell immunoglobulin and mucin-domain containing-3) and IDO (Indoleamine 2,3-dioxygenase), thereby protecting the tumours from immune-mediated elimination11. The gene discussed is CTLA4; the disease is neoplasm.