HER2-low breast cancer is defined as tumors with HER2 protein low expression without detectable gene amplification, specifically characterized as immunohistochemistry (IHC) 1+ or IHC 2+ with fluorescence in situ hybridization (FISH) negative (9).In addition to HER2 positive and HER2 low expression, the DESTINY-Breast06 study also proposed the concept of HER2 ultra-low expression. The gene discussed is ERBB2; the disease is breast cancer.