AIM:To detect germline mutations of MLH1,and investigate microsatellite instability and expression of MLH1 in tumor tissues of hereditary non-polyposis colorectal cancer (HNPCC) with two novel germline mutations,and further investigate the pathobiology of the two novel mutations of MLH1. METHODS:RNA was extracted from the peripheral blood of 12 patients from 12 different families that fulfilled the AmsterdamⅡCriteria for HNPCC.Germline mutations of MLH1 were determined by RT-PCR, followed by cDNA sequencing analysis.PCR-GeneScan analysis was used to investigate microsatellite instability with a panel of five microsatellite markers (BAT26, BAT25,D5S346,D2S123 and mfd15),along with immunohistochemical staining to detect the expression of MLH1 protein in two patients’ tumor tissues with novel mutations. RESULTS:Three germline mutations were found in four patients,one of the mutations has previously been reported,but the other two,CGC→TGC at codon 217 of exon 8 and CCG→CTG at codon 581 of exon 16,have not been reported.The two patients’ tumor tissues with novel mutations had high-frequency microsatellite instability that showed more than two unstable loci,and both tumors lost their MLH1 protein expression. CONCLUSION:The two novel germline mutations of MLH1 in HNPCC families i.e.CGC→TGC at codon 217 of exon 8 and CCG→CTG at codon 581 of exon 16,are very likely to have pathological significance. AIM: To detect germline mutations of MLH1, and investigate the microsatellite instability and expression of MLH1 in tumor tissues of hereditary non-polyposis colorectal cancer (HNPCC) with two novel germline mutations, and further investigate the pathobiology of the two novel mutations of MLH1. METHODS : RNA was extracted from the peripheral blood of 12 patients from 12 different families that fulfilled the Amsterdam II Criteria for HNPCC. Germline mutations of MLH1 were determined by RT-PCR, followed by cDNA sequencing analysis. PCR-GeneScan analysis was used to investigate microsatellite instability with a panel of five microsatellite markers (BAT26, BAT25, D5S346, D2S123 and mfd15) along with immunohistochemical staining to detect the expression of MLH1 protein in two patients’ tumor tissues with novel mutations. RESULTS: Three germline mutations were found in four patients, one of the mutations has previously been reported, but the other two, CGC → TGC at codon 217 of exon 8 and CCG → CTG at codon 5 81 of exon 16, have not been reported. The two patients’ tumor tissues with novel mutations had high-frequency microsatellite instability that showed more than two unstable loci, and both tumors lost their MLH1 protein expression. CONCLUSION: The two novel germline mutations of MLH1 in HNPCC families ieCGC → TGC at codon 217 of exon 8 and CCG → CTG at codon 581 of exon 16, are very likely to have pathological significance.