Mixed Molecular Subtypes Coexist in Estrogen Receptor Heterogeneous Primary Breast Cancers

This study demonstrates mixed Lum-A and Lum-B molecular subtypes within ER-intermediate primary breast cancers and reveals that ER- tumor cell populations have molecular features of endocrine resistance but chemotherapy sensitivity. These findings may explain the worse clinical outcomes of patients with ER-heterogeneous breast cancers and suggest benefit from combined endocrine and chemotherapy strategies.