MRC1 and colorectal cancer: Herein, we have reported four cases of the Lynch syndrome proband patients with simultaneous or heterozygous colorectal cancer and extraintestinal tumors (one each undergoing MMR germline and systemic mutation detection, IHC staining for MMR protein expression, or MSI analysis) to reveal the molecular characteristics and correlations of MMR gene mutation, MMR protein expression, and MSI among different tumors in the same patient.