Human breast cancer epithelial cell line MCF-7 (estrogen receptor (ERα-positive) and triple-negative human breast cancer cell line MDA-MB-231 (ER-, PR-, HER-2-negative) were cultured in DMEM/F12 containing 10% fetal bovine serum (FBS) (Life Technologies) at 37 °C with 5% CO2 air. The gene discussed is ESR1; the disease is breast cancer.