ERBB2 and breast cancer: The patients were eligible for the study if they had HER2-positive (centrally confirmed), metastatic, locally recurrent, or unresectable breast cancer, measurable or non-measurable disease; had received ≤ 1 hormonal regimen for metastatic breast cancer prior to randomisation, disease-free interval of at least 12 months since prior neoadjuvant treatment, and left ventricular ejection fraction (LVEF) ≥ 50% at baseline.