Recently, the expression level of HER2 in pathologically classified HER2-low (HER2 IHC 1 + or 2+, ISH non-amplified) cancers has been a topic of discussion after the DESTINY-Breast04 trial showed positive results in the treatment of HER2-low metastatic cancers with HER2-targeting antibody-drug conjugates (ADC) [38]. The gene discussed is ERBB2; the disease is metastatic malignant neoplasm.