Objective:Germline mutations in mismatch repair(MMR)genes cause Lynch syndrome(LS).LS is an inherited disease,and an important consequence of MMR deficiency is microsatellite instability(MSI)phenotype.MSI phenotype influences the efficacy of5 fluorouracil(5-FU)chemotherapy.Reproducible,cost effective,and easy to perform laboratory tests are required to include MSI detection in routine laboratory practice.Evaluation of CAT25 as monomorphic short tandem repeat sequence enables CAT25 to be an efficient screening tool among hereditary nonpolyposis colorectal cancer(HNPCC)patients compared with other methods used currently.Methods:Based on Amsterdam II criteria,31 patients in 31 families were shortlisted from a total number of 1,659 colorectal cancer patients.MSI status was examined in these patients using CAT25 and a commercially available Promega MSI five-markerbased detection system as well as immunohistochemical(IHC)staining of four important MMR proteins.Patients were scored as high microsatellite instable(MSI-H),low(MSI-L),or stable(MSS).MSI status determined by CAT25 single mononucleotide marker was compared with that of five mononucleotide markers,Promega commercial kit,and IHC method.Results:MMR protein deficiency was observed on 7/31 probands using IHC methodology and 6/31 categorized as MSI-H using commercial kit or CAT25 single marker.The sensitivity and specificity of the CAT25 single marker were the same as those detected by five-marker Promega commercial kit in our patients.Conclusions:Based on our results,the performance of the CAT25 single mononucleotide marker for MSI status determination in our HNPCC patients is the same as that of the five-marker-based commercial kit. Objective: Germline mutations in mismatch repair (MMR) genes cause Lynch syndrome (LS). LS is an inherited disease, and an important consequence of MMR deficiency is microsatellite instability (MSI) phenotype. MSI phenotype influences the efficacy of 5 fluorouracil (5-FU ) chemotherapy. Reproducible, cost effective, and easy to perform laboratory tests are required to include MSI detection in routine laboratory practice. Evaluation of CAT25 as monomorphic short tandem repeat sequence enable CAT25 to be an efficient screening tool among hereditary nonpolyposis colorectal cancer (HNPCC) Patients compared with other methods used currently. Methods: Based on Amsterdam II criteria, 31 patients in 31 families were shortlisted from a total number of 1,659 colorectal cancer patients. MSI status was examined in these patients using CAT25 and a commercially available Promega MSI five- markerbased detection system as well as immunohistochemical (IHC) staining of four important MMR proteins. Patients were scored as high mi MSI status determined by CAT25 single mononucleotide marker was compared with that of five mononucleotide markers, Promega commercial kit, and IHC method. Results: MMR protein (MSI-L), or stable deficiency was observed on 7/31 probands using IHC methodology and 6/31 categorized as MSI-H using commercial kit or CAT25 single marker. sensitivity and specificity of the CAT25 single marker were the same as those detected by five-marker Promega commercial kit in our patients. Conlusions: Based on our results, the performance of the CAT25 single mononucleotide marker for MSI status determination in our HNPCC patients is the same as that of that of the five-marker-based commercial kit.