Effect of HSD3B1 (1245C) Gene Mutation on Treatment of Stage I-III Breast Cancer

This study is examining whether a genetic variation in the HSD3B1 gene affects how well hormone-blocking therapy works in postmenopausal women with early-stage, hormone-sensitive breast cancer before surgery. **You may be eligible if:** - You are a postmenopausal woman 18 years or older - You have been diagnosed with stage I–III breast cancer that is estrogen receptor-positive (ER+) and HER2-negative, confirmed by biopsy - Your tumor is at least 1 cm in size - Your tumor has a Ki67 level of at least 10% (indicating moderate to high tumor activity) - You are a candidate for surgery **You may NOT be eligible if:** - You are premenopausal (unless on ovarian suppression therapy) - Your cancer is HER2-positive - Your cancer is ER-negative - You are pregnant or planning to become pregnant - Your tumor is too small to provide sufficient tissue after the diagnostic biopsy Talk to your doctor to see if this trial is right for you.