Fig. 1.

Acquisition and characteristics of circulating cell-free DNA (ccfDNA). Whole blood acquired through venipuncture is centrifuged to separate plasma from buffy coat and erythrocytes (a). ccfDNA derived from apoptosis is present in plasma as various multiples of nucleosomes—DNA wrapped around a histone core with a linker fragment of DNA (~10 bp) joining adjacent nucleosomes (a). The relative quantity and fragment length distribution of ccfDNA is shown in (b), where the most abundant fragment length corresponds to the length of the mononucleosome. The fragment lengths of circulating tumour DNA (ctDNA) tend to be shorter than ccfDNA (c); however, there is substantial overlap. Enrichment of ctDNA has generally focused on isolation of fragment lengths < 150 bp to improve the ratio between ctDNA and ccfDNA