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Objective To investigate the expression and mutations of DNA mismatch repair ( MMR) of Lynch syn-drome in the members of a Chinese Hui family. Methods The propositus was male, 37 years old, the forth generation, Hui nationality, had rectal poorly differentiated adenocarcinoma. There were six generations existing ( Hui nationality), and a total of 14 patients with colorectal cancer were found including 7 living cases. The peripheral blood and tumor tissue DNA was collected from 6 cases of patients and 5 healthy controls. The microsatellite instability ( MSI ) was detected through DNA sequencing, the expression of MMR protein was measured using immunohistochemistry, and MMR gene muta-tions was detected by PCR amplification. Results The MSI detection of tumor tissues in the family members demonstrated MSI-H.The immunohistochemistry showed that MLH1 protein was negative. Two new missense mutation sites ( c.264G>T and c.265G>T) in the first exon of MLH1 gene were found in 6 patients and 1 normal individual, and c.264G>T mu-tation led to early termination of MLH1 protein translation on this site. Conclusion There is MLH1 mutation in Hui family with Lynch syndrome, that is the missense muntation site c.264G>T in the first exon of MLH1 gene. [ABSTRACT FROM AUTHOR]