ERBB2 and metastatic colorectal cancer: Human epidermal receptor growth factor 2 (HER2)-amplified metastatic colorectal cancer (mCRC) comprises ~2–3% of patients with mCRC1, 2 and represents a molecularly distinct subgroup of colorectal cancer that is characterized by a worse prognosis and resistance to anti–epidermal growth factor receptor (EGFR) monoclonal antibodies3–6.