ERBB2 and breast cancer: AYAs are more likely than older women with breast cancer to present with unfavourable biology (aggressive breast cancer subtypes including triple-negative [TN] or human epidermal growth factor receptor 2 [HER2]–positive) and are more likely to be found at advanced stage, when it is often more aggressive and difficult to treat, resulting in a 39% lower survival rate compared with older women [2–4].