ESR1 and cancer: For instance, hormone, chemo and HER2/neu molecular-targeting therapies are widely used based on the expression states of estrogen receptor (ER), progesterone receptor (PR), Ki67 and the HER2/neu expression status in cancer cells (Sorlie et al. 2003; Bauer et al. 2007; Rastogi et al. 2008; Perez et al. 2010; Abd El-Rehim et al. 2005; Mattie et al. 2006).