ESR1 and breast cancer: It has been shown that women diagnosed with estrogen receptor-positive (ER+)/progesterone receptor-positive (PR+) tumors are more responsive to hormonal treatment and have a better prognosis than those diagnosed with estrogen receptor-negative (ER−)/progesterone receptor negative (PR−) tumors, indicating etiologic heterogeneity of hormone-receptor defined subtypes of breast cancer [1].