Breast cancer is subtyped into different molecular groups based on immunohistochemistry (IHC) testing for the expression of the two hormone receptors: estrogen receptor (ER) and progesterone receptor (PR) along with human epidermal growth factor receptor 2 (HER2) from formalin fixed paraffin embedded (FFPE) tissue samples taken from core biopsy of the primary breast tumour [2, 3]. Here, ESR1 is linked to breast cancer.