Human Epidermal Growth Factor Receptor 2 (HER2), also known as ErbB-2, is a cell membrane-bound receptor tyrosine kinase of the EGFR/ErbB family, encoded by the ERBB2 gene. It is crucial for cell growth and differentiation signaling. Uniquely, HER2 is an orphan receptor lacking a direct ligand, but forms highly active heterodimers with other ligand-bound ErbB family members, significantly amplifying oncogenic signals. Aberrant amplification or overexpression of the HER2/neu proto-oncogene is a key oncogenic driver. In breast cancer, approximately 25-30% of patients exhibit HER2 overexpression, leading to a more aggressive disease phenotype with faster growth, increased metastasis, and often reduced chemotherapy sensitivity. This correlates with poorer prognosis and makes HER2 a critical prognostic, predictive biomarker, and a major therapeutic target.