Still clinical markers commonly used to classify breast cancer are tumor size, lymph node involvement, histological grade, expression of steroid receptors (estrogen receptors [ER]/progesterone receptors [PR]) and human epidermal growth factor receptor 2 (Her2), but recent studies have focused on more detailed biological characteristics to improve patient risk stratification and the benefit to side-effect ratio from a specific treatment modality [3–5]. Here, ESR1 is linked to breast cancer.