Redefining HER2 in Breast Cancer: From Conventional Positivity to Low and Ultralow in the New Era of Antibody-Drug Conjugates.

Human epidermal growth factor receptor 2 (HER2) has evolved from a poor prognostic factor to one of the most impactful predictive biomarkers in oncology. The introduction of trastuzumab established HER2 as a binary determinant of therapy, with HER2 immunohistochemistry (IHC) becoming the treatment-guiding diagnostic assay. However, the emergence of antibody-drug conjugates (ADCs), particularly trastuzumab deruxtecan, has challenged this paradigm by demonstrating activity in tumors with low and even ultralow HER2 expression. This review outlines the evolution of HER2 testing, from its historical discovery and early assay variability to the ADC era, where categories such as HER2-low and HER2-ultralow have emerged. We highlight key challenges: poor reproducibility at the lower end of IHC scoring, instability of HER2 status across timepoints and between primary and metastatic tumors, and no consistent biological distinction between low, ultralow, and null. Efforts to improve reliability-including structured training, high-sensitivity assays, RNA-based methods, and artificial intelligence-assisted pathology-are summarized. Finally, we reconsider the therapeutic implications of ADCs, with the question of whether the benefit parallels HER2 expression or extends into HER2-null disease. HER2 exemplifies how biomarker interpretation evolves with drug development. As new ADCs target additional antigens, pathologists must balance trial-driven categories with biologic reproducibility to ensure diagnostics remain aligned with therapeutic advances.