HER2-Low Gastric and Gastroesophageal Junction Adenocarcinoma: From Assessment to Treatment Strategies

Human epidermal growth factor receptor 2 (HER2) dysregulation contributes to tumorigenesis in gastric and gastroesophageal junction adenocarcinomas (GC/GEJ). HER2 overexpression has been associated in multiple cohorts with aggressive behavior and poor outcomes. While HER2 amplification has long guided therapy in HER2-positive disease, antibody–drug conjugates (ADCs) have shifted attention toward the HER2-low category, typically defined as immunohistochemistry (IHC) 1+ or IHC 2+ with negative in situ hybridization (ISH). This narrative review integrates evidence from the peer-reviewed literature, current testing recommendations, and registered clinical trials. It clarifies practical issues in HER2-low assessment and maps the evolving therapeutic landscape of HER2-targeted ADCs including rational combination strategies that may extend benefit beyond conventionally HER2-positive tumors. A cross-tumor perspective contrasts GC/GEJ testing and biology with the breast cancer paradigm and summarizes the importance of HER2-low expression in non-gastric malignancies. Finally, we discuss the therapeutic strategies in HER2-low GC/GEJ and highlight key safety and monitoring considerations for HER2-directed ADCs.