ESR1 and breast cancer: The expression levels of these hormonal receptors allow for classification of BC into subtypes, with tumors lacking in ER, PR, and HER2 expression are classified as triple-negative breast cancer (TNBC).4 Unlike other cancer types, research has yet to confirm targeted therapy for TNBC, and the specific molecular underpinnings of the aggressiveness displayed by TNBC are not well understood.