Breast cancer has long been recognized as a heterogeneous entity, with distinct subsets characterized by differences in tumor biology and response to therapy. With the advent of molecular profiling, we have gained a further appreciation of the heterogeneity of this complex disease. While the last decade has seen advances in the treatment of hormone receptor (HR) and human epidermal growth factor receptor 2/erb-B2 (HER2)-positive breast cancers, outcomes for women with estrogen receptor (ER)-, progesterone...