Background: DNA mismatch repair is a process in which normal cells maintain their original genomic information avoiding errors in mismatched nucleotides or base modifications from recombination processes. The failure of this process could drive carcinogenesis due to increased mutation rate as well as high microsatellite instability. It is estimated that the global average prevalence of dMMR/MSI-H across tumor types and stages is approximately 16%, with higher rates reported for early stages. However, the prevalence for colorectal (CRC) and non-colorectal cancers for this biomarker in Mexico has not been reported. Objective:To explore the prevalence of dMMR in metastatic CRC (mCRC) and non-CRC (gastric, esophageal, and endometrial cancer) from a private Mexican institution. Methods: In this retrospective study, the dMMR status of 215 samples were analyzed from a primary database of stored biopsies from patients diagnosed with metastatic CRC, gastric, esophageal, or endometrial cancers between January 2017 and December 2020. 65 records had information about MMR status. Samples were analyzed using a commercial kit to detect dMMR (BioSb). Prevalence of dMMR was stratified by age and gender and expressed as frequencies. Statistical analysis was conducted in SPSS. Results: dMMR was found in 18.5% (N=27) endometrial cancer cases, 12.7% (N=150) of CRC cases, and 8.3% (N=24) of gastric cancer cases. However, it was not detected in any cases of esophageal cancer (N=14). In CRC, we found a higher percentage of incidence in men than in women, 15.6% (n = 14/90) vs. 8.3% (n = 5/60) respectively. By age groups, the highest total percentage (20%) was observed among those 41-50 years old. The age group with the lowest dMMR rate was 31- 40 yo, with 7.7%. All cases of CRC patients over 91 yo (n=3), were pMMR. Conclusions: Even though this data is not representative for the entire Mexican population, it provides preliminary prevalence dMMR estimates for some key cancers. Additional research is needed to give the limited sample sizes available for non-CRC.