To address the issue of diversity, HER2 genetic heterogeneity (GH) was defined by the College of American Pathologists (CAP) as the presence of more than 5% but less than 50% of infiltrating tumor cells with a HER2/CEP17 ratio >2.2 when using a control probe (or >6 HER2 signals per cell when using a probe for HER2 only) [14]. The gene discussed is ERBB2; the disease is neoplasm.