ESR1 and breast cancer: The estrogen receptor α (ERα+) status of breast cancer cells is utilized as both a prognostic and predictive marker of cancer outcomes; triple negative (TN) breast cancers lack the ERα, progesterone (PR) and HER2 receptors and are inherently more difficult to treat, whereas ERα+ tumors respond well to therapies and are uniquely suited to adjunct use of anti-estrogen therapy [23].