ERBB2 and breast cancer: The human epidermal growth factor receptor 2 (HER2) gene encodes a receptor tyrosine kinase that is amplified or overexpressed in approximately 15% of human breast cancers and is a potent driver of oncogenic transformation and breast tumorigenesis (Akiyama et al. 1986; Al-Kuraya et al. 2004; Dawood et al. 2010; Fiore et al. 1987; Muller et al. 1988; Slamon et al. 1987).