ERBB2 and breast cancer: The role of human epidermal growth receptor 2 (HER2) in breast cancer (BC) has been widely studied since the late 1980s [1–4] and has recently been established as a biomarker of poor prognosis in BC patients [5, 6], whereas in gastric cancer (GC), the role of HER2 as a biomarker of poor prognosis remains unclear [7].