The study protocol was approved by the Riverside Research Ethics Committee (Imperial College Healthcare NHS Trust; REC reference number: 07/Q0401/20) and conducted in accordance with Good Clinical Practice Guidelines and the Declaration of Helsinki. All patients gave written informed consent prior to participation. We recruited 112 patients with radiologically-confirmed metastatic breast cancer (Table 1). 20 ml of blood was taken into EDTA- tubes (BD Biosciences) and processed to plasma for cfDNA (21 (link)) and 7.5ml blood was taken into CellSave preservative tubes for CTCs capture and enumeration with the CELLSEARCH® CTC Test (Janssen Diagnostics) as described previously (22 ). CA15-3 and ALP results were obtained from patient notes.