Inclusion Criteria:

- Willing to return to enrolling institution for follow-up

- Willing to complete required testing

- Ability to complete questionnaire by themselves or with assistance

- Female (sex that was assigned at birth)

- Ipsilateral intact breast with histology confirmation of atypical ductal or lobular
hyperplasia, or LCIS, within the last 12 months, whether surgically excised or not.

- Eastern Cooperative Oncology Group (ECOG) performance status ≤1

- Willingness to agree to use ONE effective form of birth control (abstinence is not an
allowed method) prior to study entry and for the duration of study participation, and
for 2 months following the last dose of study medications. Effective birth control
methods are: copper IUD [intrauterine device], diaphragm/cervical cap/shield,
spermicide, contraceptive sponge, condoms. Women of childbearing potential must have a
negative pregnancy test within five days before starting study medications. Should a
participant become pregnant or suspect she is pregnant while participating in this
study; the participant should inform the study physician immediately.

- Willingness to avoid exposing breast skin to natural or artificial sunlight (i.e.
tanning beds) for the duration of the study.

- Participants must have acceptable organ and marrow function as defined below within 30
days of randomization: judged by treating physician's evaluation of baseline
laboratory data.

- Negative urine pregnancy test, if of childbearing potential. and / or FSH to verify
menopausal status.

Exclusion Criteria:

- Clinically suspicious mass/lesions Breast cancer in the past 5 years.

- Prior thromboembolism within last 5 years (history of varicose veins and superficial
phlebitis is allowed) Current pregnancy or lactation History of other prior breast
cancer-specific therapy within the previous 2 years (chemotherapy, anti-HER2 agents,
endocrine agents, everolimus, CDK4-6 inhibitors).

- Cytotoxic chemotherapy for any indication in last 2 years.

- Prior use of SERMS or AIs including tamoxifen, raloxifene, anastrozole, letrozole, or
exemestane for prevention or therapy within 5 years.

- Exogenous sex steroid, including oral contraceptive pill use within 1 month prior to
research core needle biopsy (CNB).

- Use of vaginally administered estrogens and hormone coated IUD such as Mirena is
permitted History of any prior ipsilateral breast radiotherapy. Previous unilateral
radiation of the contralateral side is allowed.

- Skin lesions on the breast that disrupt the stratum corneum (eg eczema, ulceration).

- History of endometrial neoplasia

- Current smoker. Cessation for at least 6 weeks

- Current users of potent inhibitors of tamoxifen metabolism. The potent inhibitors of
tamoxifen metabolism are: bupropion, cinacalcet, fluoxetine, paroxetine, quinidine.

- Participants may not be receiving any other investigational agents within 90 days of
enrollment or during this study.

- History of allergic reactions to tamoxifen.

- Uncontrolled intercurrent illness that in the judgement of the treating physician
would make them unsuitable for study participation

- Anticoagulation meds and clinical concern for discontinuing meds for study research
biopsy.

- Identification of a clinically suspicious mass on examination.

Note: Other protocol defined Inclusion/Exclusion criteria may apply.

Eligibility last updated 10/12/22. Questions regarding updates should be directed to the study team contact.