ESR1 and breast cancer: Human epidermal growth factor receptor 2 (Erbb2/HER2, hereafter called HER2) and estrogen receptor alpha (ESR1) are the most important prognostic and treatment predictive biomarkers in breast cancer (BC) and they are the most widely used therapeutic targets for this disease [1], [2].The HER2 oncogene is amplified in 15–20% of all invasive BCs, leading to overexpression of the gene.