The researchers also compared their findings with previous studies analyzing gene-expression changes in aging brain, muscle, and kidney tissue in humans, as well as in the tissues of flies and worms. The researchers found one theme universal to gene expression and aging: the slowing of a cell's energy factory. In each of these species, expression of genes related to energy production dropped twofold by the end of an animal's life span--about 2 years in mice and 80 years in humans. "This is the only common property of growing old between the four different animals," says Kim. "Maybe that should alert us to say there is something unavoidable to getting old."

However, researchers found few other similarities, raising the issue of how good of a model mice--or any of the other standard lab animals--provide for studying human aging. For example, studies in humans and other animals suggest that length of telomeres--repetitive strips of DNA at the end of each chromosome--is linked to aging. But researchers didn't find changes in expression of telomere-related genes in mice as they aged. "I wouldn't say that this means that model organisms can't be used to study aging in humans," says Promislow. "It does suggest there is a lot more going on."

Kim's analysis is likely the first of many analyses that will take advantage of the new database, dubbed AGEMAP. Scientists still need to figure out the function of the genetic networks implicated in aging.

"The scale of this study is phenomenal," says Promislow. "In some ways, this shows us where things are likely to be headed in coming years in terms of the kinds of experiments people will do to understand the genetic basis of complex traits."