ESR1 and breast cancer: Abnormally activated ErbB Family receptors (EGFR [ErbB1], HER2 [ErbB2], ErbB3 and ErbB4) and estrogen receptors (ER) are frequently implicated in breast cancer, making these potential therapeutic targets (Hurvitz and Pietras 2008; Ciardiello and Tortora 2008).