ERBB2 and breast cancer: With the advent of gene expression profiling technologies, researchers have been able to dissect the genetic and phenotypic variability among tumors and differentiate breast cancer into four molecular subtypes based on the presence or absence of the estrogen and/or progesterone hormone receptors (HR) and overexpression of the human epidermal growth factor 2 (HER2) protein: luminal A (HR+/HER2-), luminal B (HR+/HER2+), HER2-enriched (HR-/HER2+) and basal-like (HR-/HER2-) [2–5].