A Study to Evaluate Exemestane Tablets Combined With Ovarian Function Suppression/Ablation in Treatment of Premenopausal Breast Cancer Patients With CYP2D6*10 Mutations （STEP）

Inclusion Criteria:

* Providing informed consent forms voluntarily before screening evaluation;
* For Chinese premenopausal women, her estradiol level must be within the premenopausal level, or the patient meet the following 4 criteria in the prior 6 months: No chemotherapy, Regular menstruation, No use of hormonal, contraceptives, No use of hormone for treatment or for temporary amenorrhea caused by chemotherapy, the estradiol level tested within 8 months after the last dose of chemotherapeutics is within the premenopausal level;
* Patients with invasive breast cancer which has been confirmed by histological examination;
* Complete removal of tumor by surgery without local residual;
* Neoadjuvant chemotherapy before surgery is permitted if the surgery for primary breast cancer is performed within 12 weeks without any further adjuvant chemotherapy, or adjuvant chemotherapy is completed within 8 months;
* Estrogen receptor (ER) and/or progesterone receptor (PR) positive: If the patient has more than one breast tumor lesions, each tumor lesion should be ER and/or PR positive.
* Her-2 negative;
* Genotyping test performed by the central laboratory designated by sponsor with the results confirmed as CYP2D6\*10T/T gene mutation.
* Women of childbearing age with negative serum pregnancy test result, and agreeing to adopt highly efficient non-hormonal contraception measure throughout the study;
* Subject without major organ dysfunction, and with normal heart, liver, kidney, lung and other major organ function.

Exclusion Criteria:

* Inflammatory breast cancer;
* Breast cancer patients with supraclavicular lymph nodes metastasis;
* Patients with enlarged internal mammary lymph nodes (except for patients with negative pathologic findings);
* Ovariectomy which is not specified in the study;
* Patients with ovary protection during the chemotherapy;
* Concomitant use of other aromatase inhibitors (not Exemestane);
* Received major surgery which was unrelated to breast cancer within four weeks before randomization, or the patients had not yet fully recovered from such surgery;
* Pregnant or lactating women;
* Known active hepatitis B or hepatitis C or HIV;
* Having difficulty in swallowing oral preparations and gastrointestinal dysfunction;
* Recently had severe and uncontrolled systemic diseases (e.g.: cardiovascular disease, lung disease, or metabolic disease, venous thrombosis with clinical significance);
* Currently or previously suffering from other malignant tumors (except for skin basal cell carcinoma or squamous cell carcinoma, carcinoma in situ of cervix which had been fully treated), unless a radical treatment had been done with the evidence of no-recurrence or metastasis in nearly five years;
* Allergic to any study drug or any ingredients of drug;
* Patient with poor compliance or other conditions which makes the patient unsuitable to participate in this study judged by the investigator.