Negative results were seen with trials using metformin and aromatase inhibitors in hormone receptor (HR)-positive breast cancer [11], metformin/doxorubicin/cyclophosphamide in human epidermal growth factor receptor 2 (HER2)-negative breast cancer [12], and metformin and erlotinib in patients with metastatic triple negative breast cancer [13]. Here, CYP19A1 is linked to breast cancer.