Historically, breast tumor classification and therapeutic decisions have relied on immunohistochemical (IHC) techniques for characterizing biomarkers such as estrogen receptor (ER), progesterone receptor (PR), and the epidermal growth factor receptor 2 (HER2), as described in the review by Ma and colleagues. However, these markers have been found to be inadequate for fully predicting a patient’s response to a given breast cancer treatment such as endocrine therapy.
