ERBB2 and breast cancer: Despite advances in therapies, the prognosis of patients with metastatic breast cancer (MBC) remains poor, with a median time to progression on first-line therapy of 9.7 months for triple-negative breast cancer (TNBC) and 25.3 months for HR + , HER2 negative breast cancer and a median overall survival (OS) of 23 months for TNBC and 63.9 months for HR + , HER2 negative breast cancer [3–5].