ESR1 and breast cancer: Guidelines for Estrogen and Progesterone Receptor Testing in Breast Cancer recommend the measurement of ER in biopsied breast tumor tissue using a standardized, semiquantitative immunohistochemistry test that is used to diagnose cancer as the rationale basis for predicting which patients may benefit from endocrine therapy (although in practice, clinicians may prescribe treatment regimens that include endocrine therapy to breast cancer patients when it is not clear that it will be efficacious).