Human breast cancer cell lines MCF-7 and ZR-75-1 were purchased from American Type Culture Collection (ATCC) and were re-authenticated by short tandem repeat profiling (15 (link)). MCF-7 was cultured in Dulbecco's Modified Eagle's Medium supplemented with 10% FBS and 1% penicillin/streptomycin. ZR-75-1 was cultured in Improved Minimum Essential Medium (IMEM) supplemented with 5% FBS and 1% penicillin/streptomycin. LCC2 and AK-47 are two tamoxifen resistant cell lines derived from MCF-7 and ZR-75-1 respectively, kindly provided by Dr. Robert Clarke (Georgetown University Medical School, Washington, D.C.) (16 (link)) and used in our previous study (15 (link)). LCC2 and AK-47 were both cultured in IMEM supplemented with 5% charcoal-stripped FBS and 1% penicillin/streptomycin. MCF-10A was purchased from ATCC and cultured in MEGM™ Mammary Epithelial Cell Growth Medium (Lonza) with 100 ng/ml cholera toxin, 0.5 mg/ml Hydrocortisone, 20 ng/ml EGF, 10 μg/ml Insulin, 5% FBS and 1% penicillin/streptomycin. All the cell lines used in this study have been passaged and kept fewer than 6 months after the re-authentication or thawing.