ERBB2 and breast cancer: With the advent of new anti-HER2 antibody-drug conjugates (ADCs) and other targeted agents, along with promising results from clinical trials testing novel agents in the HER2 setting, recognizing HER2-low expression in breast cancer (BC) (defined as HER2 immunohistochemistry [IHC] score of 1+ or 2+ without HER2 gene amplification) is becoming increasingly significant [1–6].