The Guide to Biomarker Testing in Breast Cancer

Biomarker testing shapes nearly every treatment decision in breast cancer, from initial subtyping through resistance monitoring at progression. Hormone receptor and HER2 status remain the foundation of first-line treatment selection, but the biomarker landscape has expanded considerably—HER2-low and HER2-ultralow classifications now extend antibody-drug conjugate eligibility beyond traditionally HER2-negative disease, while PIK3CA, AKT1, and ESR1 testing increasingly guide therapy selection and switching in the hormone receptor-positive, metastatic setting. This slideshow reviews the biomarkers oncologists should be testing for across the diagnosis-to-progression continuum, along with current ASCO and ASCO/CAP testing recommendations.