The patient received adjuvant CAPEOX-based chemoradiotherapy and was followed up for 24 months.<h4>Results</h4>Histopathology confirmed two synchronous primary adenocarcinomas: the ascending colon tumor was poorly differentiated adenocarcinoma with deficient MMR (dMMR; MLH1-, PMS2-; MSH2+, MSH6+), BRAF V600E wild-type, and negative MLH1 promoter methylation in both tumor tissue and peripheral blood; the rectal tumor was moderately-to-poorly differentiated adenocarcinoma with proficient MMR (pMMR; MLH1+, PMS2+, MSH2+, MSH6+), adjacent to a sessile serrated lesion (SSL). Here, MSH6 is linked to rectal neoplasm.