ESR1 and breast cancer: Inferior outcomes for TNBC are, in large part, attributable to the common presence of micrometastatic disease at diagnosis and the absence of efficacious, molecularly targeted therapies such as the antiestrogenic aromatase inhibitors for estrogen receptor–positive breast cancer and the HER2-directed agents such as trastuzumab for breast cancers expressing high (3+ score) levels of HER2.