The guillotined French queen left behind a lock of hair that had been clipped when she was a child. Her mitochondrial DNA was extracted from the hair and the analysis has been published in a genetics journal. A genealogist in Hawaii later discovered a genetic match when he entered the string of numbers of his own genetic signature into a search engine. Tom Bopp discovered that his DNA matches that of the beheaded Marie Antoinette.
Just who Mr. Bopp's and the monarch's shared ancestor might be is unknown. But that doesn't stop Ms. Bopp from telling her husband to "eat cake" when he expresses premature concern about what's for dinner.
You can read more about the Tom Bopp/Marie Antoinette connection as well as other stories about DNA testing in a New York Times article at http://www.nytimes.com/2006/06/11/weekinreview/11harmon.html