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In September 2022, the US Food and Drug Administration (FDA) expanded approval of the legacy Roche monoclonal antibody 4B5-based immunohistochemistry (IHC) assay to identify patients with HER2-low breast cancers predicted to respond to trastuzumab deruxtecan (T-DXd), based on findings from the DESTINY-Breast04 and DESTINY-Breast06 clinical trials. However, this repurposing of a legacy biomarker assay raises significant clinical and technical validation concerns. The legacy HER2 IHC assay was originally developed to identify tumors with HER2 overexpression (3+) resulting from gene amplification, which leads to substantially higher HER2 receptor expression than in low/ultralow expression tumors. The current application to distinguish HER2-low and HER2-ultralow tumors from truly HER2-negative tumors represents a fundamentally different biological and clinical purpose, yet the assay's analytical and clinical validation for this new purpose remains incomplete. Critical gaps include the lack of established analytical sensitivity and specificity for identifying 1+ HER2 cases, poor reproducibility of pathologist scoring at low HER2 levels, and the absence of alternative methodologies for orthogonal validation. While more sensitive quantitative approaches (such as AQUA) may detect additional low HER2 expression cases missed by conventional IHC, increased analytical sensitivity does not automatically translate to clinical utility. Furthermore, the ASCO/CAP guidelines cutoffs were developed for HER2 overexpression detection and may not be the best choice for HER2-low/ultralow identification without clinical validation and determination of fit-for-purpose analytical specifications. We examine the current challenges of repurposing legacy HER2 IHC biomarker assays for HER2-low detection, evaluating the precedent of other repurposed IHC assays (ALK, CD30, and PD-L1), and emphasizing the necessity for proper technical and clinical validation before widespread implementation. We conclude that prospective clinical trials are essential to establish clinically meaningful cutoffs and analytical specifications appropriate for patient selection in HER2-low disease.