Original Article This work is licensed under the Creative Commons Attribution 4.0 License. Published by Pacifc Group of e-Journals (PaGe) Microsatellite instability in Endometrial carcinoma. Introduction Microsatellites are repeat sequences of several DNA bases. They are generally used for paternity testing and other forensic investigations as they are found both in exon and intron regions. As they have repeat structure they are more prone to slippage and thus any error in these regions that take place during replication are repaired by DNA mismatch repair genes (MMR) which are mainly MLH1, MSH2, MSH6, PMS2. [1] In tumors, microsatellite repeat number is different from that in normal tissues which is known as microsatellite instability. MSI has been related to carcinogenicity in various tumors including Lynch syndrome. The role of MSI in colorectal carcinoma has been very well studied with a prevalence of 12-24% and it is documented that colorectal tumors with MSI behave in a different way clinically when compared to tumors without MSI. [2,3] MSI positivity in colorectal carcinoma is associated with favourable prognosis. Studies of MSI in endometrial carcinoma have been very few. In a study by Caduff et al it was found that endometrial tumors positive for MSI have a high grade and a poor prognosis. [4] On the basis of tumor histology, biology and clinical features endometrial cancers are divided into two major types. Type 1 endometrial cancer is more common (70-80%), is hormone-sensitive and occurs commonly in women exposed to estrogen. It is generally associated with a higher level of tumor differentiation and has a good prognosis. Type 2 endometrial cancer is less common (20-30%) and includes serous and clear cell histology. It is characterized by poor level of differentiation and has a higher probability of myometrial invasion with poor prognosis and behaves more aggressively. In addition to these histological subtypes endometrial carcinoma is also classifed on basis of molecular alterations one of which is MSI. In 1998 the National Cancer Institute (USA) recommended panel of 05 MSI markers for the determination of MSI. The tumor is called MSI-high if it shows instability in at least 02 markers out of 05; MSI-low if 1 out of 05 and MSI-stable if none. MSI association with prognosis in endometrial carcinoma is not clearly understood. The purpose of our study is to add upon existing knowledge and to compare clinical characteristics and prognosis in endometrial tumors with and without MSI. Materials and Methods Data Collection: 40 patients who were treated for endometrial carcinoma at tertiary care hospital in western Maharashtra between January 2017 and March 2020 were included in the study. The institute ethical clearance was sought before initiation of study. Review blocks were used for study of MSI after permission from head of the institution. The histological type was classifed using World Health Organisation criteria and surgical staging Sharanjit Singh, Prateek Kinra*, Aman Kumar and Onkar Singh Hothi Dept of Pathology, Armed Forces Medical College ABSTRACT Background: Endometrial carcinoma is the commonest gynaecological malignancy in the western countries with a standardised incidence of 8 per 100000 women. In India and Southeast Asia, the incidence of endometrial carcinoma is low but it is increasing due to increasing prevalence of obesity, diabetes, early menarche, late menopause, late marriage and declining birth rate. The study aimed to identify the role of microsatellite instability in endometrial carcinoma. Although MSI has been studied extensively in colorectal carcinoma there have been very few studies in southeast asian region regarding association of MSI and prognosis in endometrial carcinoma. To the best of our knowledge this is frst study in India. Method: A Descriptive study in which 40 patients of endometrial carcinoma were studied. MSI was detected using immunohistochemistry (MSH-1, PMS-2, MSH-2, MSH-6). Statistical Analysis was done using SPSS software and fsher exact test was used to calculate p value. p value less than.05% was considered signifcant. Results: Overall prevalence of microsatellite instability was 40%. Microsatellite instability was associated with higher tumor grade, myometrial invasion>50% and presented in initial stages compared to microsatellite stable tumors. Conclusion: Our study showed statistically signifcant association between microsatellite instability and Figo staging, tumor grade and myometrial invasion. Keywords: Endometrial Carcinoma, Microsatellite Instability, Lynch Syndrome, Colorectal Carcinoma, Carcinogenesis. DOI: 10.21276/APALM.2907