HER2-LOW: Biomarker, not Driver

In this opening volley the faculty sweep away two decades of myth-making and clarify that 1+ or 2+ staining is not a biologic or oncogenic throttle but merely a molecular door handle for trastuzumab deruxtecan. Low and ultra-low disease, they illuminate, behave like their HER2-null cousins; the antigen is useful only so that an antibody–drug conjugate can hitch a ride. That conceptual pivot reframes HER2 expression as a delivery door rather than an oncogenic fuse, liberating pathologists to grade staining on a four-tier spectrum and clinicians to target the protein without over-interpreting its meaning.