Adults who were at least 18 years old with locally recurrent, unresectable, or metastatic HER2-positive adenocarcinoma of the breast (without brain metastases), a normal left ventricular ejection fraction (≥ 50%), an Eastern Cooperative Oncology Group (ECOG) performance status (PS) of 0 or 1, and who had not received previous chemotherapy or anti-HER2 therapy for their metastatic disease were randomized to receive docetaxel, trastuzumab and pertuzumab or docetaxel, or trastuzumab and placebo. This evidence concerns the gene ERBB2 and breast adenocarcinoma.