Breast cancers are a phenotypically diverse group of diseases and are usually divided into five surrogate intrinsic subtypes based on molecular characteristics of the presence or absence of oestrogen receptors (ERs), progesterone receptors (PRs), human epidermal growth factor receptor 2 (HER2), and Ki-67, a marker of proliferation (Table 1). The gene discussed is ERBB2; the disease is breast carcinoma.