Inclusion Criteria:

1. Female, age 40 or older at the time of enrollment
2. Most recent MBI examination, performed within 3 years of enrollment, showed moderate or marked background parenchymal uptake
3. Mammogram performed within 24 months prior to enrollment that is available for comparison
4. Willing and able to return for MBI following 30 days of low-dose tamoxifen
5. If able to become pregnant
   1. Negative pregnancy test within 48 hours prior to study MBI exam(s)
   2. Agrees to avoid pregnancy during the study and for at least 2 months after study participation ends, by abstinence, barrier method, or nonhormonal contraception.
6. Understands and signs the consent form

Exclusion Criteria:

1) Evidence of suspected breast disease as defined by positive findings or recommendation for short-interval follow-up on most recent breast imaging (including mammography, MBI, MRI, ultrasound, etc.) not yet resolved prior to enrollment 2) Breast biopsy or breast surgery performed 6 months prior to enrollment 3) Bilateral breast implants or status post-bilateral prophylactic mastectomy 4) Pregnant or lactating 5) Current or recent use (within 6 months prior to enrollment) of any of the following drugs:

1. Systemic hormonal therapy (oral or transdermal patch formulations)
2. Hormonal contraception (oral, transdermal, implanted, or injected formulations)  Hormonal IUDs are allowed
3. Selective estrogen receptor modulators (tamoxifen, raloxifene, or toremifene)
4. Aromatase inhibitors (anastrazole, letrozole, or exemestane)
5. GnRH analogs
6. Prolactin inhibitors
7. Androgens or antiandrogens
8. Anticoagulants or "blood thinners" (warfarin, heparin, rivaroxaban and other novel anticoagulants)
9. Drugs known to be strong inhibitors of CYP2D6, the major P450 enzyme that metabolizes tamoxifen, including:
   • bupropion (Wellbutrin)
   • fluoxetine (Prozac)
   • paroxetine (Paxil)
   • quinidine (Quinidex) 6) Personal history of any type of malignancy, with the exclusion of non-melanoma skin cancer, diagnosed prior to enrollment 7) Personal history or strong family history of blood clots in legs or lungs (also known as deep vein thrombosis or pulmonary embolism) 8) Personal history of transient ischemic attack (TIA) or cerebrovascular accident (CVA) 9) Active proliferative disorders of the endometrium such as atypical hyperplasia, history of active endometriosis, unresected polyps 10) Any type of retinal disorders or severe cataract 11) Current smoker 12) Known carrier of BRCA1 or BRCA2 genetic mutation or known DNA repair defect 13) Has high blood pressure