Abstract: ObjectiveTo evaluate the significance of the expression of MLH1,MSH2,PMS2 and MSH6 proteins in hereditary non polyposis colorectal cancer cases in Yunnan region. MethodsUsing current three criteria commonly used in China and abroad to select HNPCC 19 tumor tissue cases from 13 families in Yunnan region,and these tumor tissue were assessed by immunohistochemical detection of MLH1,MSH2,PMS2 and MSH6 proteins. Results In the 19 tumor tissues cases, the expression loss rates of MLH1, MSH2, PMS2 and MSH6 proteins were 30.77%, 38.46%, 23.08% and 15.38% respectively.And in these case of absent of MMR protein, 2 cases were lost MLH1 and PMS2 proteins at the same time; 3 cases lost MSH2 and MSH6 at the same time.The total expression loss rates of four MMR proteins was 84.62%. ConclusionThe suspected HNPCC cases in the Yunnan region had loss of expression of the MMR (MLH1, MSH2, PMS2,MSH6).The immunohistochemical detection of MMR proteins can be used as an effective technique in screening HNPCC pedigrees.