ESR1 and breast cancer: While the introduction of tamoxifen and trastuzumab have improved outcomes for patients with hormone receptor-positive and HER2-positive cancers, respectively [6, 7], no equivalent targeted agents have yet been identified for breast cancers of the “triple negative” histological subtype (negative for estrogen receptor (ER), progesterone receptor (PR), and HER2) and the molecularly defined basal-like subtype [8, 9].