To investigate the role of SOX11 in regulating the growth of breast cancer cells, we used siRNA to inhibit SOX11 and measure two-dimensional growth in-vitro in a panel of ER-negative/HER2-negative (BT20, MDA-MB-468, HCC70, HCC1937, and MDA-MB-231), ER-negative/HER2-positive (SKBr-3, and HCC1954), ER-positive/HER2-positive (BT474), and ER-positive/HER2-negative (MCF7 and ZR751) breast cancer cell lines as well as non-transformed breast cell lines (HME-hTERT and MCF10A). This evidence concerns the gene ESR1 and breast cancer.