ERBB2 and breast cancer: In the PerELISA study in 61 evaluable postmenopausal patients with HR+/HER2+ operable BC, following two weeks of letrozole, molecular responders (Ki67 relative reduction >20% from the baseline at two weeks) continued endocrine therapy in combination with dual HER2-targeted blockade with trastuzumab and pertuzumab, whereas molecular nonresponders switched to taxane-based chemotherapy plus dual HER2-targeted blockade.