ESR1 and breast cancer: Although for clinical purposes, breast cancers are categorized based on the expression of a few key markers, such as the estrogen receptor (ER), the progesterone receptor (PR) and the human epidermal growth factor receptor 2 (HER2) whose levels are assessed using immunohistochemistry (IHC)-based methods[1], recent genomic studies have uncovered much greater heterogeneity in breast cancers.