ESR1 and breast cancer: Examining clinical factors, 48% of total patients were diagnosed with stage I breast cancer, 65% had hormone receptor-positive (ER positive or PR positive)/HER2-negative tumor subtype, 66% were negative for lymph node involvement, 35% had a tumor size of 1–2 cm, 42% had a tumor grade of II, 79% had a ductal histology, and 12% received care at a NCI-designated Cancer Center.