Breast tumors can be classified into 5 intrinsic subtypes as measured by mRNA expression,
1 with most estrogen receptor (ER)–positive tumors classified as luminal A or luminal B subtype. In general, luminal A tumors have higher ER-expression and lower tumor cell proliferation than luminal B tumors.
2,3 Approximately 1 in 4 patients with ER-positive breast cancer will develop distant metastasis and eventually die from the disease,
4,5 and it is currently not possible to accurately predict a patient’s long-term risk or the benefit of endocrine therapy.