Patients will first undergo screening, tumor measurement, and mandatory collection of core tumor biopsies for central determination of HER2 and HR status. These biopsies will be used to determine gene expression patterns once the patient is included in the study. Patients will be treated with dual HER2 blockade consisting of lapatinib and trastuzumab for a total of 18 weeks. Patients who are HR-positive will also be given endocrine therapy, letrozole or tamoxifen depending on their menopausal status, for the same 18 weeks. If tumor progression is observed by ultrasound (US) at week 6, tumors will be identified as resistant, and paclitaxel will be added to dual HER2 blockade, maintaining trastuzumab at the same original dose and reducing lapatinib dose for safety reasons. In those patients with HR-positive disease, endocrine therapy will be withdrawn in order to avoid its adverse interactions with chemotherapy.