Abstract: Abstract: Objective To investigate the distribution of the MDR1 exon12 (C1236T), exon21 (G2677T/A) and exon 26 (C3435T) gene polymorphisms in breast cancer patients, and to analyse their relationship with molecular subtypes of breast cancer. Methods The genotyping of C1236T, G2677T/A and C3435T were detected by polymerase chain reaction (PCR)- high resolution melting (HRM) method in 400 cases of breast cancer. The Hardy-Weinberg equilibrium test was used for ge⁃ netic equilibrium distribution of genotype. The molecular subtypes of breast cancer were classified based on St.Gallen Con⁃ sensus 2013. The genotype distributions of C1236T, G2677T/A and C3435T in breast cancer were analyzed. Their relation⁃ ship with molecular subtypes in breast cancer was analyzed as well. Results ① In 400 cases of breast cancer, there were 2, 3 and 2 specimens did not get genotyping results in C1236T, G2677T/A and C3435T genotype detection. The CC, CT and TT genotypes of C1236T accounted for 16.08% (64/398), 44.22% (176/398) and 39.70% (176/398). GG, GT, GA, TT and AT genotypes of G2677T/A accounted for 16.62% (66/397), 44.33% (176/397), 7.05% (28/397), 27.46% (109/397) and 4.54% (18/397). CC, CT and TT genotypes of C3435T accounted for 21.11% (84/398), 56.03% (223/398) and 22.86% (91/398) re⁃ spectively. Hardy-Weinberg genetic equilibrium testing showed that polymorphisms of C1236T, G2677T/A and C3435T had group representation (P < 0.05). ② Eleven cases of HER-2 (2+) were excluded because they were not verified by FISH de⁃tection when performed molecular subtype of breast cancer. Luminal A subtype accounted for 41.90% (163/389), Luminal B subtype accounted for 32.65% (127/389), HER-2 over-expression subtype accounted for 13.62% (53/389) and triple negative subtype accounted for 11.83% (46/389). ③ CT/TT genotype frequency of C3435T was significantly higher in breast cancer patients with Luminal A subtype than that in breast cancer patients with HER-2 over-expression subtype and triple negative subtype (χ2 =12.011, P=0.001; χ2 =13.976, P < 0.001), while there was no statistical difference in C1236T and G2677T/ A gene polymorphism between different molecular subtypes of breast cancer (P > 0.05). Conclusion C3435T gene polymor⁃ phism can explain more accurately heterogeneity of breast cancer. CT/TT genotype in different molecular subtypes of breast cancer may be more sensitive to drug treatment.

Key words: breast neoplasms, polymorphism, single nucleotide, high resolution melting, molecular subtype, C1236T, G2677T/A, C3435T