Microsatellite instability (MSI) detection I. Microsatellite instability (MSI) The DNA mismatch repair (MMR) system is a stringent proofreader that identifies and corrects mismatched DNA bases during replication.DNA mismatch repair genes play an important role in maintaining the stability of the genome. Microsatellites are highly polymorphic repetitive sequences containing 1-6 bases in the eukaryotic genome. microsatellite instability (MSI) arises from the addition or loss of simple repetitive sequences due to DNA replication errors (RER), which are associated with abnormalities in MMR-related genes. a large number of MSI are often found in the genomes of DNA MMR-deficient tumors. ii. MSI and colorectal cancer 2. 1 MSI and colorectal cancer prognosis and treatment Clinical studies have shown that stage II colon cancer patients with microsatellite hyperinstability (MSI-H) do not benefit from adjuvant fluoride therapy and have a better prognosis without fluoride chemotherapy [2]. The Chinese Colorectal Cancer Diagnostic and Treatment Protocol specifically states that MMR protein expression or MSI testing is recommended for stage II patients, and fluorouracil monotherapy adjuvant chemotherapy is not recommended for patients with MMR protein expression deficiency or MSI-H. The NCCN colorectal cancer screening guidelines state that stage II patients with MSI-H (high microsatellite instability) may have a better prognosis and will not benefit from adjuvant chemotherapy with 5-FU. 2.2 MSI and hereditary non-polyposis colorectal cancer (HNPCC) HNPCC (e.g. Lynch syndrome) is an autosomal dominant disease, and approximately 50%-60% of HNPCC families are due to defects in the DNA mismatch repair system (MMR) gene. Another marker of MMR protein deficiency is “microsatellite instability”. Considering the possibility that HNPCC is increasing in the colorectal cancer population, the NCCN panel recommends that MMR-related testing (including MSI and MMR gene mutations) should be actively considered for all colorectal cancer patients younger than 50 years of age, and for patients with HNPCC, extensive colectomy should be considered [1] and for patients with MSI-H or MMR protein expression deficiency, further MMR testing should be performed. positive patients, further MMR gene-related testing should be performed. Clinical implications ① Patients with stage II colorectal cancer with microsatellite hyperinstability (MSI-H) have a better prognosis, but do not benefit from adjuvant chemotherapy with fluorouracil drugs. ② Colorectal cancer patients with MSI-H need further MMR gene-related testing to diagnose HNPCC. Applicable population ① Stage II colorectal cancer patients ② Colorectal cancer patients under 50 years old V. Specimen type Paraffin tissue + peripheral blood