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Mice, men share 99 percent of genes

By Marsha Walton

Scientists say mice and humans descended from a common ancestor about the size of a small rat.
Scientists say mice and humans descended from a common ancestor about the size of a small rat.

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(CNN) -- When it comes to DNA, it turns out there's not that much difference between mice and men.

Mice and humans each have about 30,000 genes, yet only 300 are unique to either organism. Both even have genes for a tail, even though it's not "switched on" in humans.

"About 99 percent of genes in humans have counterparts in the mouse," said Eric Lander, Director of the Whitehead Institute Center for Genomic Research in Cambridge, Massachusetts. "Eighty percent have identical, one-to-one counterparts."

The mouse is the only mammal, after the human, whose genome has been sequenced. The rodent's genetic sequence was published in this week's edition of Nature Magazine.

Finding important human genes

To some researchers, unraveling the mysteries of the mouse's genetic blueprint is as exciting as the breakthrough in February 2001 of the sequencing of the human genome.

"Having a sequence of the human genome is good, but our ability to interpret it was limited," Lander said. "The mouse sequence provides, for the first time, an ability to determine what matters and what doesn't in the human genome."

A genome is all the genetic material in an organism, unique to every plant or animal.

For decades, the laboratory mouse has been known as the scientist's best friend. Researchers around the world study them as a way of learning about and treating human diseases, from heart disease to malaria to obesity.

Now the genetic road map of the mouse may help find answers to even more medical puzzles. Scientists say these new details will help researchers focus more rapidly on the specific genes that cause disease in humans, with the hope illnesses can be diagnosed and treated faster and more efficiently.

Reason for sharing

Scientists say mice, humans and many other mammals descended from a common ancestor about the size of a small rat from 75 to 125 million years ago. That creature lived alongside the dinosaur. While mice and humans certainly don't look much the same these days, their genetic blueprints are startlingly similar.

What's even more astonishing, scientists say, is that 90 percent of genes associated with disease are identical in humans and mice. Because new generations of mice are born just weeks or months apart, and because medical experiments with humans are usually not done for ethical reasons, mice have become valuable research tools.

In the process of comparing the mouse and human genomes, researchers also discovered 1,200 new human genes

Of course, there are also important differences. The mouse genome is about 14 percent smaller than the human version.

Mice have many more genes for smell than humans. And they have more genes to produce frequent and large litters.

But about 40 percent of the two genomes are directly aligned.

Available to all

Publication of the genome means that scientists anywhere in the world will be able to use, at no charge, a comprehensive library of gene sequences tied to their specific research.

About a year and a half ago, the private company Celera Genomics Corp. announced its nearly complete gene map of the mouse. But the company did not publish complete details of the achievement. Instead, they made it available on a fee basis to scientific subscribers.

One participant in the Mouse Genome Project is the Wellcome Trust Sanger Institute, an independent research-funding charity in the United Kingdom. Even before their research was complete, the facility handled about six million requests for genome sequences, for research into diabetes, deafness, cancer and other diseases.

More research needed

Much of the material within the genome is not yet understood, said Dr. Jane Rogers, a Wellcome Institute scientist.

"Genomes are used in different ways by different organisms, and we need to understand more how they are regulated," Rogers said. She says getting to the bottom of how genomes work is likely to unravel bit by bit for years to come.

Lander says unlocking the secrets of the mouse genome will make a huge difference in understanding the basis of disease.

"It's going to lead to a really precise encyclopedia of what is in the human genome, what's in cancer and other diseases," he said.

Other scientists now are working on genetic blueprints of the rat, cow, chimpanzee and dog.

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