ESR1 and breast cancer: While PAGES-HBC’s stratification of HER2+ER+ breast cancer patients was comparable to that of all HER2+ patients (HR 0.17, 95% CI 0.04–0.65, p = 0.012), we could not determine a significance in HER2+ER− patients, which was likely due to the small cohort size, including only 14 Group 1 and 6 Group 2 patients.