In a pilot study, we found that the MCF7 cell line-derived, ER+, estrogen-independent, and tamoxifen-resistant, MCF7-TamC3 breast cancer cells exhibit increased expression of two epigenetic regulators, HDAC5 and HDAC2, compared to the parental ER+ MCF7 breast cancer cells 15. This evidence concerns the gene ESR1 and breast cancer.