To cletermine the role of mismatch repair in the clevelopment of oral squamolls cell carcinomas (OSCCs) , the prevalence of microsatellite instability (MSI), expression of I뼈LH1 ancll띠SH2 ， ancl hypennethylation of I뻐LH1 ancl hMSH2 were explorecl. Bya panel of five markers (BAT25, BAT26 , 02S123, 05S346, ancl 017~‘250 ， the so-callecl Bethescla markers) for screening of MSI, MSI was observecl in 5 of the 15 sqllamolls cell carcinomas (33.3%). As MSI is callsecl by the clysfllnction of MMR genes, this stucly examinecl the methylation status of CpG sites in the hMLH 1 ancl hMSH2 promoters ancl the expression hMLH1 ancl hMSH2. Becallse of inappr이)riate efficiency of fonnalin-frxecl ancl paraffin-embeclclecl samples for methylation-specific PCR (MS-PCR) of hMLH1 ancl hMSH2, the role of promoter hypelmethylation 띠 the clevelopment of MSI ancl expression of hMLH1 ancl hMSH2. meùlylation was failecl to clefìne. However, loss of nllclear staining of 1ψIILH 1 ancl hMSH2 were seen in nine (69.2%) ancl four (30.7%) of 13 OSCCs, while four ancl fìve all corresponcling normal epiùlelial tisslles showecl positive nllclear stι ining ofl마ILH1 and hMSH2. These data suggest that MSI anclloss of expression of hMLH1 ancl hMSH2 play a role in the carcinc핑enesis of oral sqllamolls cell carcinomas thollgh the role of promoter hypermethylation of hMLH1 ancl hMSH2 in MSI and their expresslon IS lIncertam