A Sensitive Denaturing Gradient-Gel Electrophoresis Assay Reveals a High Frequency of Heteroplasmy in Hypervariable Region 1 of the Human mtDNA Control Region**Disclaimer: The opinions and assertions expressed herein are solely those of the authors and are not to be construed as official or the views of the United States Department of Defense, the United States Department of the Army, or the United States Department of Commerce. This paper is a contribution of the United States National Institute of Standards and Technology (NIST) and the United States Department of Defense and is not subject to copyright. Certain commercial equipment, instruments, or companies are identified in this paper to specify the experimental procedure. Such identification does not imply recommendation or endorsement by the government, nor does it imply that the materials or equipment identified are the best available for this purpose.