ESR1 and breast cancer: However, one finding of the present subgroup analysis, namely that patients with HER2‐positive breast cancer that is sensitive to endocrine therapy (as shown by their longer TTF) generally have longer OS, suggests that the addition of a CDK 4/6 inhibitor to endocrine therapy may benefit postmenopausal patients with ER‐positive HER2‐positive AMBC and who are not candidates for chemotherapy; the combination of a CDK 4/6 inhibitor plus endocrine therapy would avoid the risk of adverse effects associated with chemotherapy.