This study aims to evaluate the feasibility of plasma ctDNA mutation spectrum and clonal spectrum in late-stage metastatic breast cancer patients. Meanwhile, this study tries to establish a model of ctDNA subtyping system to evaluate the molecular load of tumor, to evaluate the curative effect comprehensively and to detect the disease progression in advance. In addition, the investigators plan to explore the clonal evolution of ctDNA in patients with progressive disease (PD), and to monitor the changes of tumor heterogeneity from the molecular level, so as to provide reference for the analysis of drug sensitivity.
