ESR1 and breast cancer: Approximately 10% to 20% of breast cancers are triple negative breast cancer (TNBC), a breast tumor subtype that is clinically negative for expression of the estrogen receptor (ER) and progesterone receptor (PR) and lacks overexpression of the human epidermal growth factor receptor 2 (HER2).[1] TNBC often has a poor prognosis due to its aggressive clinical behavior and lack of response to hormonal therapy or therapies that target HER2 receptors.