MRC1 and Lynch syndrome: While the hallmark of this disease is tumor MMR deficiency (MMRd), defined by the presence of microsatellite instability (MSI) and/or absence of MMR protein expression by immunohistochemistry (IHC), BRAF V600E wild type, and no MLH1 promoter hypermethylation, a diagnosis of Lynch syndrome requires the presence of a deleterious germline variant in a DNA MMR gene [7].