Depending on the studies, the proportion of unexplained MSI/dMMR colorectal and endometrial tumors (i.e., tumors without a pathogenic germline variant or somatic MLH1 promoter methylation), which are caused by biallelic somatic genetic alterations can range from 17% to 95% [205,220,221,222,223,224,225]. The gene discussed is MLH1; the disease is endometrium neoplasm.