For unresectable or metastatic mismatch repair deficient (dMMR) and microsatellite instability-high (MSI-H) CRC, the FDA has approved three immune checkpoint inhibitors (ICIs) targeting PD-1 (pembrolizumab, nivolumab) and CTLA-4 (ipilimumab) in 2017 and 2018, respectively [22,23]. The gene discussed is CTLA4; the disease is colorectal carcinoma.