Background: The importance of circulating DNA has been recognized since the detection of mutated oncogene products in cancer patients; however, there is little information about circulating DNA in normal human plasma. We characterized circulating DNA in normal human plasma to obtain basic information. Methods: Circulating DNA was purified from plasma samples obtained from 10 healthy donors and examined. Purified DNA was cloned and their sequence determined and analyzed. The terminal structure was examined by a labeling method. Results: The DNA levels in normal plasma samples were quite low (3.6-5.0??ng/ml). All 556 clones analyzed were independent, and obtained from various chromosomes and various regions of the gene. The mean values of their length and GC content were 176??bp and 53.7%, respectively. Their 5??? and 3??? ends were rich in C and G, respectively, and they presented as 5??? protruding forms of double-stranded DNA in plasma. Conclusion: Circulating DNA in normal human plasma is derived from apoptotic cells but not from necrotic cells. Structural characteristics of the circulating DNA might be associated with their stability in plasma. ?? 2007 Elsevier B.V. All rights reserved.
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