Gene expression profiling has reshaped our understanding of breast cancer by defining and characterizing four main intrinsic molecular subtypes: human epidermal growth factor receptor 2-enriched, basal-like, luminal A, and luminal B subtypes. Luminal B breast cancer has been reported to have lower expression of hormone receptors, higher expression of proliferation markers, and higher histologic grade than luminal A. It also exhibits worse prognosis and has a distinct profile of response to hormone therapy and chemotherapy. Although luminal cancers share similarities, the studies conducted in recent years using next-generation sequencing technology show that luminal A and B breast cancers should be perceived as distinct entities, with specific oncogenic drivers, rather than more proliferative varieties of luminal tumors. This review discusses the definition and molecular characterization of luminal B breast cancer and presents the available clinical evidence for chemotherapy and endocrine therapy patterns of response. It also provides an overview of ongoing research on molecularly targeted agents for this disease.

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