These cell lines can be classified into three different breast cancer subtypes according to the expression of ER, PR and HER2 receptor: ER-positive (MCF-7, MDA-MB-175, ZR-751 and CAMA-1), HER2-overexpressed (SK-BR-3 and HCC1569) and triple-negative subtypes (Hs578T, MDA-MB-231, BT549 and BT20). The gene discussed is ESR1; the disease is breast carcinoma.