There are well-known differences in patient outcomes and effective therapeutic options across subtypes of breast cancer (BC), defined by the status of estrogen receptor, progesterone receptor, and erb-B2 receptor tyrosine kinase 2 (ERBB2 [formerly HER2]) expression, making testing for these receptors part of the routine workup for all patients with a diagnosis of invasive BC. This evidence concerns the gene ESR1 and breast cancer.