Fraune C, Rosebrock J, Simon R, et al. - In order to generate data on intratumoral heterogeneity of mismatch repair (MMR) deficiency/microsatellite instability (MSI) in ovarian cancer, researchers performed this study. They screened 582 ovarian cancers for MMR deficiency, and 10 cases suspect for MMR deficiency were recognized out of 478 interpretable cancers. In 9 of these tumors, MMR deficiency/MSI was confirmed on repeated immunohistochemistry on large sections integrated with polymerase chain reaction-based MSI analysis. The absence of MMR deficiency/MSI was found in 34 mucinous carcinomas, 23 clear cell carcinomas, 17 malignant mixed Mullerian tumors (carcinosarcomas), and 11 mixed carcinomas. Findings revealed the presence of MSI in a substantial proportion of endometrioid ovarian cancers, but its occurrence can be noted in other tumor subtypes. Experts noted that MMR deficiency/MSI typically involved the whole tumor mass, this indicates the early occurrence of MMR inactivation in tumorigenesis in a subset of ovarian cancers.