Both ER & PR are ligand-activated transcription factors belonging to the family of nuclear hormone receptors. While ER results are the most helpful to predicting therapy response, PR is still an important marker in breast cancer due to its role in determining the functionality of estrogen receptors in human breast cancer. PR measurements are considered to work as enhancers to the predictive potential of ER test results. The predictive value is especially important when assessing pre-menopausal women with primary breast tumors. In immunohistochemical studies, both ER and PR receptor staining are localized within the nucleus. Hormone therapy has been an established therapy for breast cancer treatment for approximately 50 years. Prior to the development of hormone therapy, the traditional treatment was oophorectomy for pre-menopausal patients and high concentrations of estrogens for other patient groups. These traditional approaches have been largely replaced with anti-estrogens, aromatase inhibitors, and luteinizing hormone agonists. Research has shown that overall, tumors reactive to ER and PR regressed with hormone therapy directed treatment.