Breast cancer patients can be clinically classified into three distinct subtypes based on oestrogen (ER)-, progesterone (PR)- and HER-2-receptor assessment (Onitilo et al, 2009) including: (i) ER-positive and human epidermal growth factor receptor 2 (HER2)-normal (ER+), (ii) HER2-positve regardless of ER (HER2+), and (iii) triple receptor-negative breast cancers (TNBC). This evidence concerns the gene ESR1 and breast cancer.