ESR1 and breast cancer: The 21‐gene test (Oncotype DX Breast Recurrence Score® test; Exact Sciences Corp, Redwood City, CA) is a validated prognostic tool and predictor of adjuvant chemotherapy benefit in patients with ER+, HER2−, node‐negative, or node‐positive early breast cancer who receive 5 years of hormonal therapy.15, 16, 17, 18, 19, 20, 21