Pembrolizumab and nivolumab—antibodies targeting programmed cell death protein 1 (PD1)—are approved as monotherapies or, in the case of nivolumab, in combination with the anti-cytotoxic T lymphocyte-associated protein 4 antibody ipilimumab for the treatment of CRC with high microsatellite instability (MSI-H)/mismatch repair deficiency (dMMR) [15]. The gene discussed is PDCD1; the disease is mismatch repair cancer syndrome 1.