It has been found that hypoxia can induce the overexpression of breast cancer resistance protein and multiple drug resistance protein [23] and the activation of the transforming growth factor beta, transforming growth factor alpha and epidermal growth factor receptor, and cyclooxygenase-2 pathways [24-26] by regulating the hypoxia-inducible factor α and cancer-associated fibroblasts, thus inducing therapeutic resistance. This evidence concerns the gene ABCG2 and cancer.