CA15-3 is the most important and specific marker for breast cancer. 30%-50% of breast cancer patients have significantly elevated CA15-3 and its level is closely related to the treatment effect, which is the best indicator for diagnosis and monitoring of post-operative recurrence of breast cancer patients and observing the efficacy of treatment. When CA15-3 is greater than 100 U/ml, metastatic lesions can be considered. CA is a tumor cell-associated antigen. The commonly used CA series are: CA 125 (ovarian cancer-associated antigen); CA 19-9 (pancreatic and intestinal cancer-associated antigen); CA 15-3 (breast cancer-associated antigen). CA 153 is a breast cancer-associated antigen, which is valuable for the diagnosis and postoperative follow-up of breast cancer. CA153 is often elevated in breast cancer and has a low sensitivity of about 60% in the early stages of breast cancer. Metastatic breast cancer has a positive rate of up to 80%. In Europe, CA153 is commonly used as an adjuvant diagnostic indicator for breast cancer and is also used for postoperative follow-up to monitor tumor recurrence and metastasis. However, other tumors such as lung cancer, kidney cancer, colon cancer, pancreatic cancer, ovarian cancer, liver cancer, etc. may also be elevated to varying degrees. However, the significance of CA153 in these tumors is not significant. The waxing and waning of patient’s serum CA15-3 level parallels the change of breast cancer disease and is an important signal of recurrence and metastasis, and this signal is given earlier than the onset of clinical symptoms and the detection of recurrence and metastasis by, for example, ultrasound, X-ray or CT. It has been reported that when CA15-3 levels exceed 30u/ml, 40u/ml and 50u/ml, the sensitivity of determining the presence of postoperative local recurrence or distant metastasis in breast cancer patients is over 90%, the specificity is 95%, 99% and 100%, and the correct judgment rate is 56%, 83% and 100%, respectively. In addition, breast cancer patients with increased CA15-3 levels develop metastasis much earlier than those with normal CA15-3. According to the analysis and research, there is an altered consistency between the change of serum CA15-3 level of breast cancer patients and their local lymph nodes and distant metastases, especially those with distant metastases, their CA15-3 expression level and positive rate are significantly increased, therefore, CA15-3 has the role of monitoring breast cancer metastasis, if its serum level continues to increase, then chemotherapy, radiotherapy or change to If its serum level continues to rise, chemotherapy, radiotherapy or endocrine therapy should be started or intensified or switched to endocrine therapy.