Billionaire Microsoft co-founder Paul Allen decided to make a genetic map of the mouse brain in 2002 with a fund of $100-million.
It was declared complete on Tuesday, with fine-tuned information on 3 000 active genes which scientists have been using it regularly for more than a year.
Allen said that working with computers all his life made him appreciate the complexities of the brain. He said, "You realize that computers take a very simplistic approach to computing things."
He added, "Ever since I grew up in Seattle as a kid, I was fascinated by science." Following the advice of a group of scientists was the first project of the Allen Institute for Brain Science - a 3-D reference atlas of the genes that are active in the mouse brain.
Allen left Microsoft in 1983. He has amassed an estimated fortune of $16-billion and now makes the map freely available online at www.alleninstitute.org.
The institute said in a statement, "Since mice and humans share more than 90 percent of genes, the Allen Brain Atlas has enormous potential for understanding human neurological diseases and disorders affecting more than 50 million Americans each year."
The genes include those of Alzheimer's disease, autism, epilepsy, schizophrenia and Parkinson's disease.
Within four years, scientists working for the Atlas project have mapped over 21 000 genes. They then checked each gene to see which ones are turned on - expressed - in brain tissue.
Although each cell in an organism's body carries all the genes, not all of them are expressed, or active. Gene expression determines each cell's type and function.
Expecting only 60 to 70 genes to be expressed in the brain, Allen's team, to their surprise found that over 80 percent of the genes in the brain are active.
In situ hybridization, which uses a chemical marker such as a jellyfish fluorescence gene to show activity of a gene, was used to develop the atlas. Tissue containing cells expressing each active gene was stained, photographed and the pictures uploaded to the website, making it easy to browse.
According to the institute an average of 250 scientists looked at the site a day, with over four million hits monthly.
Allen said, "The next set of research we are going to do is focus on the neocortex - the area where most higher function occurs." He has asked for other foundations and the US government to help support the institute project.