The choice of comparator (anastrozole in coopERA; letrozole in AMEERA-4) may be relevant, but a previous WOO study in patients with ER+ invasive breast cancer found that anastrozole 1 mg and letrozole 2.5 mg (both once daily) were similarly effective in suppressing Ki67 after 14 days [16]. Here, ESR1 is linked to invasive breast carcinoma.