According to Scientists Identify Genetic Marker for Longevity, a certain patch of mitochondrial DNA is unusually common in folks who live past 100:
In a study conducted at the California Institute of Technology in Pasadena, California, researchers found that centenarians [people over 100 years old] were five times more likely than others to have the same mutation in their mitochrondrial DNA.Mitochondrial DNA, the portion of DNA located in the mitochondria or “powerhouses” of the cell, passes only from the mother to offspring.
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In the study of a group of 52 Italian centenarians, the researchers found a common mutation in the same main control region. Looking at mitochondrial DNA in white blood cells, they found that 17 percent of the 52 had a specific mutation called C150T transition, compared with only 3.4 percent of 117 people under the age of 99.