Prevalence of Mismatch Repair Deficiency in Advanced Solid Tumors (Colorectal Cancer and Non-Colorectal Cancer) in One Mexican Institution.

: Mismatch repair (MMR) status is an important prognostic and predictive indicator in cancer, distinguishing proficient (pMMR) tumors from deficient (dMMR) ones. This study aimed to determine the prevalence of dMMR in colorectal (CRC) and selected non-CRC solid tumors (gastric, esophageal, and endometrial cancers). : This retrospective study was conducted at a private health institution in Mexico City, analyzing patients diagnosed with colorectal, gastric, esophageal, or endometrial cancer from January 2017 to December 2020. dMMR prevalence was assessed using available status information and tissue samples for immunohistochemistry (IHC). Data were analyzed via SPSS, presenting results in frequencies and percentages. : Most solid tumors exhibited MSH2, MSH6, and MLH1 expression above 90%, with slightly lower levels in endometrial cancer. Esophageal cancer showed 100% pMMR. dMMR prevalence was found to be 12.7% for CRC, 8.3% for gastric, and 18.5% for endometrial cancers. Prevalence rates were similar across genders (11.1% in women and 12.9% in men), with the highest prevalence in the 41-50 age group (20%) and the lowest in the 31-40 age group (7.7%). : This study offers valuable insights into the frequency of dMMR mutations in a cohort of the Mexican population, providing a basis for further research on their prevalence in Mexico.