Breast cancer is one of the most common female cancers and known to metastasize to brain. It is acknowledged that brain prevents the entering of substances in the blood stream by blood-brain barrier (BBB) and only tiny materials can across the BBB. In terms of size, cancer cells are too large to penetrate the BBB. However, 30% of all women with metastatic breast cancer develop brain tumors. Recently, many studies have suggested that the reciprocal interaction between cancer cells and environment is important for cancer metastasis. Therefore, it is plausible that tumor microenvironment is involved in breast cancer brain metastasis. Our research is to understand the role of Interface Zone in breast cancer brain metastasis.