Liquid biopsies are poised to revolutionize cancer therapies, by enabling frequent blood-based monitoring of tumor-derived materials, as cancers evolve in response to therapeutic interventions. Nowhere is this more evident than in triple negative metastatic breast cancer, where women may undergo multiple serial courses of therapy over many years, each associated with an initial response, followed by the acquisition of drug resistance. Plasma DNA (ctDNA) mutation analyses provide a measure of tumor burden and may identify targetable mutations for small molecule inhibitors, but they do not allow ex vivo culture of intact circulating tumor cells (CTCs) for individualized preclinical drug sensitivity (“precision oncology”). Isolation of CTCs has been achieved using small blood volumes (5-10 mL), but these cells are so rare that it has not been feasible to establish clinically robust ex vivo CTC cultures. Our hypothesis is that we can massively increase the amount of blood screened through standard clinical leukapheresis (1-2 L), and in return, achieve 100+-fold increase in the number of isolated CTCs to establish routine ex vivo culture of CTCs. It is also important to note that ex vivo CTC cultures obtained by sampling 1-2 L blood will reflect the “real time” molecular subtype, genetic composition, and evolving drug sensitivity profile of an individual patient's metastatic breast cancer, including multiple lesions that together contribute to the blood-borne CTC population and the patient's overall tumor burden. We will develop a high-throughput microfluidic technology that achieves highly efficient depletion of tagged blood cells away from unmanipulated single and clustered CTCs and provides “tumor independent enrichment” performance, which is applicable to cells disseminated from any solid tumor type without making any a priori assumption on the physical and/or biological properties of tumor cells. We will also establish the microenvironmental conditions conducive to high efficiency ex vivo CTC cultures from women with metastatic triple negative breast cancer. Success would be transformative both for new drug development and for individualized patient selection among existing therapies for breast cancer patients, and others.