Koga Y, et al. - Thirty-eight cases of ≤10-mm flat colorectal carcinomas (CRCs) without an adenoma component (de novo type) were compared with 39 cases of ≤10-mm flat CRCs with an adenoma component (carcinoma in adenoma (CIA) type) by the researchers to determine their different clinicopathological features and phenotypic classification and whether CRCs were obtained from an adenoma-carcinoma sequence or were de novo in nature. In comparison with the CIA type, the de novo-type CRCs were more commonly located in the proximal colon, more generally penetrated submucosa, and more often had venous permeation. Regarding the phenotypic classification based on the immunohistochemical expressions of CD10, MUC2, and MUC5AC, the incidence of unclassified type (CD10−, MUC2− , and MUC5AC−) was markedly more commonly observed in the de novo than CIA type. Since MLH1 and PMS2 protein expressions were immunohistochemically negative mismatch repair (MMR) protein loss was judged in one de novo-type case. BRAF mutation was observed in one de novo-type case and two CIA-type cases, however, none of these cases had MMR protein loss. Hence, small-intestinal type (CD10+ and MUC5AC−) was concluded as the most common in flat CRC and the unclassified type was chiefly representative of the de novo type. In this study, small flat CRCs with BRAF mutations do not have MMR protein loss.