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By Clive Cookson

Published: October 13 2006 03:00 | Last updated: October 13 2006 03:00

Now that the human and chimpanzee genomes have been published, scientists are trying to discover the genetic factors that make the two species different. The aim is to find the stretches of DNA that have evolved most rapidly over the 5m or so years since humans diverged from apes.

A study at the University of California has identified 202 "highly accelerated regions" (HARs) which are very similar in chimps and other mammals, such as mice and rats, but distinctly different in humans. Only three of these HARs are genes in the conventional sense: stretches of DNA that code for proteins. The other 199 lie close to genes involved in growth and development but are not genes themselves.

"Most of the differences between chimps and humans are not in our proteins but in how we use them," said Katherine Pollard, who led the research at the UC Davis Genome Centre.

Some of the most important HARs appear to regulate the genes involved in brain development. But the paper describing the research in the journal PLoS Genetics concludes: "While we can pinpoint the locations of the most rapidly accelerated elements in the human genome, we cannot determine the exact cause of this acceleration with present data."