Breast cancer is the most frequently diagnosed malignancy in women and is the leading cause of cancer mortality in women worldwide. HER2 (ERBB2)-positive breast cancer comprises approximately 20 to 25% of the entire breast cancer population. ERBB2 protein overexpression or ERBB2 gene amplification in breast cancer tumors is associated with more aggressive clinical disease and poorer prognosis. Current standard of care for patients with HER2-positive early breast cancer is chemotherapy and one year of adjuvant trastuzumab, however approximately 20% of patients with HER2-positive early breast cancer will recur within 5 years after adjuvant therapy.