A map of the mouse brain down to details of individual cells has been completed, the first project of an institute funded by Microsoft Corp. (MSFT) co-founder Paul G. Allen, it was announced Tuesday.

The new Allen Brain Atlas is being made available online without cost to neuroscientists studying brain circuits and chemistry, a potential boon to cancer and other disease research because of similarities between the brains of mice and human beings, according to a statement issued by the Allen Institute of Brain Science.

"We want people to use this and make discoveries," Dr. Allan Jones, the institute's chief scientific officer, told The Seattle Post-Intelligencer.

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A formal announcement was planned in Washington, D.C., with Allen and Sens. Patty Murray, D-Wash, and Ted Stevens, R-Alaska.

Because more than 90 percent of the same genes are found in mice and humans, the mouse brain map can be compared with genetic findings related to human neurological disorders.

Moreover, the mapping project has shown that 80 percent of the body's genes are switched on in the brain, compared with 60 percent to 70 percent in previous scientific estimates, Jones said.

Neurosurgeons at Swedish Medical Center in Seattle have begun using the atlas to study the genetics of brain cancers, said neuroscientist Marc Tessier-Lavigne, an institute science adviser who discovered the molecules that trigger connections between nerve cells.

"This really just bolts us ahead in our ability to understand brain function and brain disorder," Tessier-Lavigne told The Seattle Times.

Even before completion of the mouse brain map, the institute's work had become vital to scientists delving into the genetics of multiple sclerosis, which is caused by degeneration of nerve cells, said Dr. Ben Barres, a Stanford University neurology professor.

"We use it every day," Barres said. "I hope they have enough bandwidth when everyone else starts using it."

The institute says it now gets 4 million hits a month as about 250 scientists access it daily.

When access to the institute was interrupted some time ago, "I learned how much we've come to depend upon this," Barres said.

"Graduate students and postdoctoral fellows just came pouring into my office, worried that they had been cut off," he said. "We can't imagine life without this tool anymore."

Allen donated $100 million to start the lab in 2003 and the mouse brain atlas cost $41 million, well under the $50 million that had been budgeted, Jones said.

The next project, Jones said, will be to develop a digital, three-dimensional, interactive map of the genes at work in a human brain's neocortex, the outer layer that is the seat of higher thought and emotion, using brains from cadavers as well as tissue removed during brain surgeries.

Scientists hope the brain-mapping research eventually will lead to new discoveries on brain function and disorders such as MS, Alzheimer's and Parkinson's diseases, epilepsy, schizophrenia and addiction, to cite just a few.

"There's been a lot of progress in understanding how the brain develops," Barres said. "The next frontier is figuring out the circuitry and how it actually works."