ESR1 and cancer: A great effort is ongoing to develop new strategies for treating its various forms subdivided into three classes: (I) hormone receptor-positive breast cancers that display approximately 70–80% of all cases, (II) human epidermal growth factor receptor 2 (HER2) overexpressing cancers in approximately 10–15% of all cases and (III) the remaining 10–15% of breast cancers that are defined by hormone receptor and HER2 negativity.22, 23, 24 Estrogen receptor (ER)-positive breast cancers show expression of two structurally related receptors ERα and ERβ.