Currently,breast cancer is a common malignancy in female,and previous guidelines recommended that one of the key biomarkers for breast cancer,human epidermal growth factor receptor 2(HER2),is classified as either positive or negative to guide clinicians'treatment decisions.While nearly half of breast cancer patients have low HER2 expression(IHC expression is 1+or 2+and ISH detection is negative),such patients are insensitive to traditional anti-HER2 targeted therapy.However,novel antibody-drug conjugates(ADCs)provide new targeted therapy options for breast cancer patients with low HER2 expression,challenging the traditional binary concept and arousing research enthusiasm.In the latest ASCO/CAP guidelines for HER2 detection in breast cancer,HER2-low breast cancer has been included as a clinical treatment subgroup.This article will review the definition of HER2-low breast cancer,the progress of drug therapy such as ADC,and the current challenges faced by this subgroup.