ESR1 and breast cancer: Breast cancer is grouped into 3 subtypes: hormone receptor positive and ERBB2 negative (approximately 65%), ERBB2 amplified (approximately 5%-20%), and triple negative (approximately 5%).2 The characterization of subtypes has changed how patients with breast cancer are treated.3 Because of this, testing for ER, PR, and ERBB2 expression is the recommended standard of care for all of those with a diagnosis of invasive breast cancer.4