The present disclosure in one aspect provides technologies for detection of breast cancer, e.g., early detection of breast cancer. In another aspect, technologies provided herein are useful for selecting and/or monitoring and/or evaluating efficacy of, a treatment administered to a subject determined to have or susceptible to breast cancer. In some embodiments, technologies provided herein are useful for development of companion diagnostics, e.g., by measuring tumor burdens and changes in tumor burdens in conjunction with therapeutics. In some embodiments, technologies provided herein are useful for development of companion diagnostics, e.g., by identifying biomarkers in subjects' bodily fluid samples (e.g., blood samples) that are associated with therapeutic response.