In this study, we performed genome-wide chromatin accessibility analysis on 23 breast cancer cell lines, including 2 estrogen receptor (ER)-positive/human epidermal growth factor receptor 2 (HER2)-negative (ER+/HER2−), 3 ER+/HER2+, 3 HER2+, and 15 triple-negative breast cancer (TNBC) lines. The gene discussed is ESR1; the disease is breast cancer.