The objective of this study is to compare the curative effects in patients under the age of 35 with hormone receptor positive breast cancer and high recurrent risk factors including large tumor or metastatic lymph nodes randomized to ovarian function suppression (OFS) plus aromatase inhibitors or OFS plus tamoxifen as adjuvant endocrine therapy, and explore the differences of curative effects between different subtypes to provide direct evidence for treatments of young breast cancer patients.

Inclusion Criteria:

1. Signed informed consent;
2. Eastern Cooperative Oncology Group (ECOG) performance status ≤ 2;
3. Patients pathologically diagnosed with hormone receptor positive invasive breast cancer: immunohistochemistry ER positive (≥1%);
4. Premenopausal patients with age ≤35 years (at least menstruate once for prior 3 months, assess menstruation situation before chemotherapy if have);
5. No distant metastasis;
6. Clinical stage (TNM) meets at least one of the conditions as follow: T≥2cm or at least one region of regional lymph node metastasis (including micrometastases);
7. Patients must have undergone a surgery and (neo)adjuvant chemotherapy, targeted therapy, radiotherapy (if necessary) according to Chinese Anti-Cancer Association guidelines and specifications version 2015 for diagnosis and treatment of breast cancer;
8. Indexes of hematology and biochemistry conform to following standards: HGB≥10g/dl, WBC≥4,000/mm3, PLT≥100,000/mm3, GOT, GPT, ALP≤2xULN, TBIL, DBIL, CCr≤1.5xULN.

Exclusion Criteria:

1. Pregnant or lactating women or women of childbearing potential reject contraceptive measures;
2. History of organ transplantation (including autologous bone marrow transplantation and peripheral stem cell transplantation);
3. Concurrent malignancies or history of non-breast malignancies within the 5 years prior to study entry, except for curatively treated basal cell and squamous cell carcinomas of the skin, carcinoma in situ of the cervix; peripheral nervous system disorders caused by diseases or history of obvious mental and central nervous system disorders;
4. Prior use of neo-adjuvant chemotherapy after a definite diagnosis;
5. Nervous system disorders caused by diseases or obvious mental disorder, which would affect patients right to consent and compliance, or make patients in critical condition;
6. Serious hepatopathy (e.g., cirrhosis, etc.), nephropathy, respiratory illness, digestive illness or poorly controlled diabetes;
7. Cardiac illness with obvious abnormal EKG or clinical symptoms, namely congestive heart failure, coronary atherosclerotic heart disease with obvious symptoms, arrhythmia difficult to control by drugs, or history of myocardial infarction within the 12 months prior to study entry or class III/IV of cardiac function;
8. Concurrent treatment in another investigational trial;
9. Sensitivity or contraindication to any of the study medications.