The anti-programmed cell death protein 1 (PD1) monoclonal antibodies nivolumab and pembrolizumab, either alone or in combination with the anti-cytotoxic T-lymphocyte antigen 4 (CTLA-4) monoclonal antibody ipilimumab, are indicated in the setting of mismatch repair deficiency. This evidence concerns the gene PDCD1 and mismatch repair cancer syndrome 1.