Accurate non-invasive biomarkers are urgently needed to identify which patients with hormone receptor positive (HR+) breast cancer will respond to neoadjuvant endocrine therapy. Lack of direct knowledge of the endocrine sensitivity of each patient’s breast cancer impedes optimal, tailored therapy. Without a more personalized approach, many women and men will continue to suffer from the current morbidity and mortality of breast cancer. The overall objective of the proposed clinical trial is to investigate the ability of quantitative, hybrid functional imaging for assessing hormonal sensitivity, estrogen receptor (ER) functional inhibition, and early response to neoadjuvant endocrine therapy. The long-term goal is to develop functional imaging approaches to directly test tumor sensitivity to endocrine therapy in breast cancer patients for individualized treatment plans and improved outcomes. The proposed research will investigate early changes in expression of a classic estrogen-regulated target gene as a surrogate measure of endocrine sensitivity: progesterone receptor (PR) using a progestin-based radioligand, 21- [18F]fluorofuranylnorprogesterone (FFNP) and quantitative simultaneous breast positron emission tomography/magnetic resonance imaging (PET/MRI). The central hypothesis is that FFNP uptake in primary breast tumors will show dynamic changes in response to presurgical endocrine therapy, which will correlate with treatment response and exceed inherent technical variability. The proposed clinical trial is a prospective, single-center study that will enroll women with newly diagnosed ER+/PR+/HER2- invasive breast cancer who will undergo simultaneous breast PET/MRI with FFNP before and after a short course of endocrine therapy prior to surgical excision. The study aims to determine 1) the efficacy of FFNP PET/MRI for predicting response to presurgical endocrine therapy and 2) the quantitative reliability of FFNP breast PET/MRI. The proposed research is innovative because it will use functional imaging with simultaneous breast PET/MRI to improve the success of neoadjuvant endocrine therapy. Imaging treatment-induced changes in estrogen-regulated signaling events, using FFNP PET/MRI, will have a significant positive impact by enabling early assessment of endocrine therapy response mediated through ER before changes in tumor size can be measured using conventional techniques such as mammography, ultrasound, and palpation. Once validated, this approach can easily be integrated into the preoperative evaluation of patients with primary HR+ breast cancer to individualize neoadjuvant and adjuvant treatment plans for improved patient outcomes.