Histology of Functional Density in Premenopausal Breast

Inclusion Criteria: * Ages 40 - 55 years * Considered premenopausal or peri-menopausal * Screening mammogram performed at Mayo Clinic Rochester within one year prior to the current MBI study which demonstrates * Negative/benign assessment (BIRADS category 1-2) or Additional imaging recommended (BIRADS category 0), where the subsequent diagnostic workup results in a final negative/benign assessment (BIRADS category 1-2). * No proliferative benign lesions (e.g. fibroadenomas) identified * Heterogeneously dense or extremely dense parenchyma (BIRADs density category 3 or 4) * MBI performed less than 3 days prior to biopsy demonstrating either marked FD or photopenic FD in an area of mammographically dense tissue amenable to biopsy. * No use of exogenous hormonal drugs (e.g. hormonal contraceptives or sex steroid hormones) within six months prior to study biopsy * Negative pregnancy test or surgically sterilized (for patients in whom a study MBI will be performed) Exclusion criteria: * Using any estrogen receptor modulating drugs (e.g., tamoxifen, raloxifene) or any aromatase inhibitors within six months prior to study biopsy. * Prior diagnosis of bilateral breast cancer. * Diagnosis of breast cancer in either breast within 3 years prior to study MBI. * Diagnosis of any cancer, except breast cancer (see #3) or non-melanomatous skin cancer, for which patient has not been disease free for at least 5 years. * Current breast symptoms * Breast implants * Known allergy to local anesthetic. * History of bleeding complications from prior interventions * Current use of anticoagulants (e.g., Coumadin or other blood thinners) * Major medical condition