This molecular typing defines luminal (ER-positive), HER2-type (HER2 overexpressing) and basal-like (often ER and PR and HER2-negative, so called triple-negative) breast cancers, and thus tumor types that respond to specific therapies such as endocrine therapy for luminal and anti-HER2 agents for HER2 type cancers. This evidence concerns the gene ESR1 and breast cancer.