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AIM: To investigate genetic instability of gene BRCA1 at locus D17S855, and their relationship with clinicopathological characteristics of gastric cancer in Chinese population.METHODS: Microsatellite instability (MSI) and loss of heterozygosity (LOH) of gene BRCA1 at locus D17S855were compared between 37 samples of gastric cancer and corresponding non-cancerous gastric tissue.RESULTS: MSI at locus D17S855 was positive in 7of 37 samples of gastric cancer (18.95%). MSI had a close relationship with TNM staging but no relation with lymph node metastasis, histological type or tumor differentiation. MSI positive frequency in TNM Ⅰ + Ⅱ (31.58%, 6/19) was much higher than that in TNM Ⅲ + Ⅳ (5.56%, 1/18), (P ＜ 0.05). LOH positive rate was 18.92% (7/37). LOH had no relationship to histological type, tumor differentiation or lymph node metastasis, but LOH positive rate in TNM Ⅲ +Ⅳ was 33.33% (6/18), much higher than that in TNM Ⅰ + Ⅱ ( 5.26%, 1/19), (P ＜ 0.05). BRCA1 protein was expressed in 14 of 37 samples of gastric cancer. The positive rates of BRCA1 protein in TNM Ⅰ + Ⅱ and TNM Ⅲ + Ⅳ were 57.89% and 16.67%, respectively, (P ＜0.05). The positive rate of BRCA1 protein was 77.78% in high differentiation samples, 30.77% in middle differentiation and 12.50% in lower differentiation samples, (P ＜0.05).CONCLUSION: MSI of BRCA1 gene could be used as a molecular marker in early phases of sporadic gastric cancer in Chinese population. LOH occurs at later period of gastric cancer, therefore, it could be used as prognostic factor.