Background\ud Genomic DNA sequences in cell-free plasma are biomarkers of cancer prognosis, where characteristic changes in methylation of tumour suppressor or oncogene DNA regions are indicative of changes in gene activity. Also, cell-free fetal DNA can be distinguished, by its methylation status, from the maternal DNA in the plasma of pregnant women, hence providing DNA biomarkers for the proposed minimally-invasive diagnosis of fetal aneuploidies, including Down's syndrome. However, the production and clearance of cell-free DNA from plasma in relation to its methylation status, are poorly understood processes.\ud \ud Methods\ud We studied the methylation status of DNA derived from the imprinted GNAS1 locus, in cell-free plasma DNA of healthy adults. Heterozygotes were identified that carried the SNP rs1800905 in the imprinted region. The parent-of-origin-dependent DNA methylation was analysed by bisulfite conversion, followed by cloning and sequencing.\ud \ud Results\ud Genomic DNA molecules derived from both the methylated, maternal, allele and the unmethylated, paternal, allele were found in plasma. Methylated and unmethylated DNA molecules were present in equal numbers.\ud \ud Conclusions\ud Our data indicate that the methylation status of a DNA sequence has no effect on its steady state concentration in the cell-free DNA component of plasma, in healthy adults.\ud \u
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