To classify HER2-low breast cancer accurately, for all patients with historical HER2-negative diagnosis (i.e., HER2 0, HER2 1+, and HER2 2+ with ISH–) (N = 591), we rescored their historical HER2 IHC slides via rigorous methods according to the latest 2018 ASCO/CAP guideline11 (Fig. 1a). The gene discussed is ERBB2; the disease is breast cancer.