Zarei S, et al. - Researchers aimed at determining the clinicopathologic, immunohistochemical (IHC), and molecular genetic characteristics of a group of cases with mixed morphologic features of high-grade serous carcinoma (HGSCA) and low-grade serous carcinoma (LGSCA). HGSCA and LGSCA are currently classified as mutually exclusive diseases based on morphology and molecular pathogenesis. Cases with mixed morphologic features of HGSCA and LGSCA are termed as indeterminate grade serous carcinoma (IGSCA) by researchers in comparison with groups of HGSCA and LGSCA. For the detailed morphologic study, 27 LGSCA and 19 IGSCA were selected using the World Health Organization classification criteria. Selection of 13 classic HGSCA, 19 classic LGSCA, and 19 IGSCA was done for p53 and BRAF V600E IHC and molecular genetic testing by next-generation sequencing. In IGSCA, they identified the architectural patterns of invasion of LGSCA, but with higher-grade nuclear characteristics focally and a mitotic index intermediate between LGSCA and HGSCA. IHC or sequencing revealed mutant TP53 in few cases in the IGSCA group (4/18, 22.2%), mutant BRAF non-V600E in 1 case, and NRAS mutation in 1. Identical mutations were observed in the low-grade and high-grade areas when present. Similar to the classic HGSCA group, long-term survival was reported in the IGSCA group. IGSCA is thus identified to be a rare and potentially clinically aggressive variant of serous carcinoma. Findings suggest distinct morphologic, but heterogeneous molecular features, including low frequency of TP53 and BRAF mutations, in these serous carcinomas indicating that these rare tumors may have a distinct pathogenesis pathway compared with classic HGSCA and classic LGSCA.