According to this definition (Vance et al., 2009), HER2 heterogeneity ranges between 5% (Vance et al., 2009) and 15% (Ohlschlegel et al., 2011) of total cases tested and seems to be most frequent (up to 27%) in breast carcinomas with an equivocal (2+) HER2 score (Ohlschlegel et al., 2011). The gene discussed is ERBB2; the disease is breast carcinoma.