The rapid accumulation of mutations in cancers and the resulting clonal heterogeneity frequently lead to generation of drug-resistant populations of cancer cells. Examinations requiring a tissue biopsy are invasive, making real-time monitoring of the disease difficult. One alternative to support tissue-based testing is fluid biopsy. Malignant cells release to the patient's circulation DNA, which contains somatic mutations and epigenetic changes characteristic of each cancer. Sufficiently sensitive methods of investigation enable the detection of tumor DNA in the blood sample, whereby it can be utilized e.g. in the monitoring of disease progression.