Over the last decade, molecular approaches, including multigene assays for predicting prognosis and treatment response, have entered into the clinical arena of breast cancer care.1, 2, 3, 4, 5, 6, 7, 8 All of these multigene assays have been shown to have some prognostic and predictive value in certain subgroups of estrogen receptor (ER) positive breast cancer patients.3, 4, 9 Oncotype DX® (ODX) is the most widely used of these multigene assays. Here, ESR1 is linked to breast cancer.