In contrast, ESR1 mutation prevalence is only 4–5% in recurrent breast cancer after prior adjuvant AI (including recurrence while on adjuvant AI) [12, 14, 15], 1.5–7% after neoadjuvant AI [16, 17], and less than 1% in ET-naïve MBC [7, 10, 14]. This evidence concerns the gene ESR1 and breast cancer.