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Background: There are obviously ethnic differences between the UGT1A1 gene polymorphisms. Due to the difference of genetic background and environment, the treatment with colorectal cancer patients of Guangxi Zhuang should not completely follow the Euramerican or Chinese han patients. The study aimed to explore the correlation of UGT1A1 gene polymorphism of Guangxi Zhuang metastatic colorectal cancer (mCRC) with irinotecan -based chemotherapy, in order to develop an individualized irinotecan regimen for mCRC patients of Guangxi Zhuang.Methods: From June 2013 and June 2015, a total of 406 patients of Guangxi who were histologically diagnosed as metastatic colorectal cancer with 102 patients of this cohort with three generations of Zhuang, and 86 patients that conformed to inclusion and exclusion criteria were competitively enrolled. The distribution of UGT1A1 gene polymorphism was analyzed-retrospectively in all patients. Pyrosequencing method was used to detect the UGT1A1*28 and*6 gene polymorphism in the 86 Guangxi Zhuang mCRC patients. After first-line chemotherapy with FOLFIRI regimen, the relationship between gene polymorphism of UGT1A1 and adverse reactions, and efficacy of Irinotecan were analyzed with χ2 test and Kaplan-Meier method.Results: UGT1A1*28 wild-type (TA6/6), heterozygous mutant (TA6/7) and homozygous mutant (TA7/7) accounted for 69.8, 30.2 and 0%, respectively. UGT1A1*6 wild type (G/G), heterozygous mutation type (G/A) and homozygous mutant (A/A) accounted for 76.7%, 20.9 and 2.3%, respectively. UGT1A1*28 TA6/7 type could increase the risk of grade 3~4 diarrhea (p = 0.027), which did not increase the risk of grade 3~4 neutropenia (p = 0.092). UGT1A1*6G/A and A/A type could increase the risk of grade 3~4 diarrhea and neutropenia (p = 0.001; p = 0.017). After chemotherapy with FOLFIRI, there was no significant difference in response rate (RR) (p = 0.729; p = 0.745) or in median progression-free survival (mPFS) between the wild-type, mutant treatment of UGT1A1*28 and UGT1A1*6 (7.0 m vs 7.4 m, p = 0.427; 6.9 m vs 7.0 m p = 0.408).Conclusions: The distribution of UGT1A1*28 and UGT1A1*6 gene polymorphism in Guangxi Zhuang patients were differed from the existing reported of European people and Chinese Han population. The UGT1A1 gene polymorphism with irinotecan chemotherapy-associated diarrhea and neutropenia were closely related. There was no significant association between UGT1A1 gene polymorphism and therapeutic efficacy of irinotecan. [ABSTRACT FROM AUTHOR]