In breast cancer clinical research, menopause is defined by different criteria. Since menopause is critical to the selection of endocrine agents for breast cancer, it is clearly defined in the 2006 NCCN guidelines. 1. previous bilateral oophorectomy; 2. age ≥ 60 years; 3. if age < 60 years. The following two conditions must be present to define menopause: (1) amenorrhea ≥ 12 months without chemotherapy, TAM, tolmetinol treatment or ovarian suppression intervention; (2) FSH and estrogen levels in the menopausal range; 4. If the patient develops amenorrhea during TAM or tolmetinol and the patient is < 60 years of age, only FSH and estrogen levels in the menopausal range can be defined as menopause. Chemotherapy often leads to amenorrhea in premenopausal patients, which is largely determined by the composition of the chemotherapy regimen and the age of the patient. However, studies have found that chemotherapy-induced amenorrhea does not imply ovarian dysfunction, and many patients with chemotherapy-induced amenorrhea still have estradiol levels at premenopausal levels. It should be particularly emphasized that when AIs are used in premenopausal patients, they cause gonadotropin secretion in patients in a feedback manner, and the application of letrozole for 3-7 days after the menstrual cycle can effectively induce ovulation in the ovaries. Therefore the definition of menopause is very important for the selection of endocrine drugs and the criteria must be strictly controlled.