Predicting Response to Neoadjuvant Endocrine Therapy (Neo-PREDICT)

The goal of this clinical trial is to determine how the duration of hormone blocking (endocrine) therapy given prior to surgery (called "neoadjuvant" treatment) affects breast cancer. The main questions the trial aims is answer are: 1. How breast cancer responds to endocrine therapy given prior to surgery? 2. To predict tumor pre-operative endocrine prognostic index (PEPI) score for subjects enrolled in cohort B or C Participants with early-stage breast cancer (Stage I-III) who are eligible for Neoadjuvant Endocrine Therapy (NET) will be enrolled in the study. Participants will: * receive endocrine therapy as part of regular care for breast cancer * consent to samples of blood and tissue evaluation to determine how endocrine therapy effects the tumor * participate in this research anywhere from 2 weeks to 1 year, depending on duration of endocrine therapy and when surgery will be performed