HER2-Low Breast Cancer: What the New Category Means

A new middle category

Historically breast cancer was classed as HER2-positive or HER2-negative by immunohistochemistry and in-situ hybridization. The HER2-low group, defined as weak incomplete staining, sits between those extremes and is now recognized because it responds to antibody-drug conjugates.

Why it matters for treatment

• HER2-positive tumors have long used trastuzumab-based therapy.
• HER2-low tumors, previously lumped with negative disease, can now benefit from HER2-directed antibody-drug conjugates in the metastatic setting.
• HER2-negative remains the residuum with the fewest HER2-targeted options.

Implication for staging and planning

Accurate receptor testing at diagnosis is essential because the HER2-low label changes the systemic therapy menu. Staging still follows the same TNM and prognostic group framework.