MSH6 and rectal cancer: Subsequently, the NCCN guidelines (v1.2021) for colon cancer (24) and rectal cancer (25) recommend that all newly diagnosed CRC patients should be tested for MMR proteins expression (including MLH1, MSH2, MSH6 and PMS2) or MSI (including BAT25, BAT26, D5S346, D2S123 and D17S250) status.