ESR1 and breast carcinoma: [1] Estrogen receptor (ER) or progesterone receptor (PR)-positive breast cancers make up approximately three-quarters of all invasive breast cancers. [2] Because hormone receptor (HR) status is a powerful predictor of the efficacy of endocrine treatment, anti-hormonal therapy is given to HR-positive patients as adjuvant treatment after curative surgery or palliative treatment.