ERBB2 and maternal uniparental disomy of chromosome 20: Human epidermal growth factor receptor 2 (HER2) overexpression and/or amplification is present in approximately 15% to 20% of metastatic breast cancers (MBCs) [2–4], and HER2-positive metastatic breast cancer (HER2+ MBC) has historically been associated with an aggressive course of disease, high rates of recurrence, and poor overall survival (OS) [5].