To determine whether FKA specifically inhibits the growth of breast cancer cells, including HER2-overexpressing breast cancer cells, an immortalized normal breast epithelial cell line, MCF10A, ER-positive (MCF7) and ER-negative (MDA-MB-468) breast cancer cell lines with less HER2 expression, and ER-positive (MCF7/HER2) and ER-negative (SKBR3) breast cancer cell lines with HER2-overexpression were examined. This evidence concerns the gene ESR1 and breast cancer.