These findings suggest that HIF-2α is not only one of the hypoxia inducible factors, but also a novel potential bio-marker which could predict worse chemotherapy effectiveness in invasive ductal carcinoma of breast; and indicate that hypoxia promoted the multidrug resistance of breast cancer cells by hypoxia/HIF-2α/ABCG2/MDR pathway. This evidence concerns the gene ABCG2 and breast cancer.