Abstract
Tumor data from the ABCSG 5 trial of chemotherapy versus endocrine therapy for premenopausal ER(+) breast cancer supports molecular subtyping by Ki-67 IHC as a prognostic marker. But while this tissue was handled uniformly, Ki-67 testing overall is unstandardized, complicating clinical utility. Increasing potential biomarkers herald more challenges in biomarker validation.See related article by Bago-Horvath et al., p. 5682.