The Long and Winding Road for Breast Cancer Biomarkers to Reach Clinical Utility.

Tumor data from the ABCGS5 trial of chemotherapy versus endocrine therapy for premenopausal ER+ breast cancer supports molecular subtyping by Ki-67 immunochemistry as a prognostic marker. But while this tissue was handled uniformly, Ki67 testing overall is unstandardized, complicating clinical utility. Increasing potential biomarkers herald more challenges in biomarker validation.