ERBB2 and breast cancer: Breast cancer patients with tumours overexpressing human epidermal growth factor receptor 2 (HER2), or triple-negative breast cancer (TNBC) that lacks expression of oestrogen and progesterone receptors (ER and PR, respectively) and does not have amplification of HER2, have a disproportionately high incidence of brain recurrence and tend to develop brain metastases early after initial diagnosis (Heitz et al., 2009).