ESR1 and breast cancer: This picture has recently been complicated further by the introduction of gene profiling approaches [8] and by the widespread application of a novel BC classification based on the immunohistochemistry (IHC) phenotypic patterns identified by a few protein biomarkers, namely ER-α, PgR, HER2, epidermal growth factor receptor (EGFR), and low-molecular-weight cytokeratins [9].