ERBB2 and breast cancer: Human epidermal growth factor receptor 2 (HER2) plays an important role in cell growth and differentiation.221 It affects the activation of the MAPK pathway and PI3K pathway, which are associated with tumorigenicity.222–225 Approximately 25% of breast cancers overexpress HER2.226–231 Studies have shown that eccDNA is involved in HER2 amplification in breast cancer.