Based on the expression of these proteins breast cancer is classically divided into five subtypes; luminal A (ER+, PR+, HER2−), luminal B (ER+, PR−, HER2+), HER2 positive (ER−, PR−, HER2+) and basal-like and triple-negative breast cancer (TNBC) breast cancers (ER−, PR− and HER2−), which are two similar but distinct subtypes of aggressive breast cancers. This evidence concerns the gene ESR1 and breast cancer.