ESR1 and breast cancer: Estrogen receptor-α (ER-α) is a key transcriptional regulator that plays critical roles in normal breast development and breast tumorigenesis.1, 2, 3, 4 Approximately 70% of breast cancers are ER-α positive and are treated with targeted antiestrogen therapy by using effective ER blocking agents,5, 6, 7 such as tamoxifen and fulvestrant.