To address this question we performed a direct comparison study of the methylation status of SRY (sex-determining region Y)-box 17 (SOX17), Cystatin M (CST6) and Breast cancer metastasis suppressor 1 (BRMS1) in primary tumours, EpCAM-positive pooled CTC-fractions and ctDNA in well characterized matched clinical samples of breast cancer patients. Here, CST6 is linked to breast cancer.