ERBB2 and breast cancer: One of these studies, the PerELISA phase II trial, evaluated a strategy of 5-cycles of neoadjuvant letrozole, trastuzumab and pertuzumab in 44 patients with early-stage HER2-positive/hormone receptor-positive breast cancer with endocrine sensitivity disease, selected on the basis of Ki67 reduction after 2-week letrozole exposure.