|Brain metastasis accounts for significant morbidity and mortality associated with many cancers, due to limited choices of therapeutic options and a lack of specific predictive measures. This devastating complication is initiated by a rare subset of the circulating tumor cells (CTCs) shed into the blood stream. Thus, analyzing CTCs from patient blood samples has potential for predicting and monitoring brain metastasis because of the non-invasive nature that allows repeated sampling. However, we know nothing about the molecular properties in those CTCs that drive the incidence of brain metastasis due to the challenge of analyzing the limited amount of patient-derived CTCs. To address this problem, we recently pioneered the ex vivo culture of CTCs isolated from breast cancer patients, enabling the detailed functional analysis of these cells for the first time. Using this unique cell resource, we identified brain metastasis-initiating CTCs, and discovered candidate signatures promoting brain metastasis. We hypothesize that CTCs have the potential as a monitoring tool for brain metastasis. The overall objectives of this proposal are to investigate the brain metastasis microenvironment, analyze the CTC dynamics before and after brain metastasis surgery, and compare the molecular features of CTCs with brain metastasis. This proposal could contribute to our understanding of the CTC biology specifically for brain metastasis and developing predictive and monitoring measures of brain metastasis.