ERBB2 and breast cancer: In light of HER2-low BC, and moreover, the proven efficacy of treatment with the new ADC outside of HER2-positive tumors defined as such by current testing guidelines, new methods of HER2 testing using gene protein assays and next-generation sequencing (NGS) may help to overcome the problem of HER2 heterogeneity, using single-slide HER2 assessment to delineate patients who will benefit from anti-HER2 treatment [105].