It's now possible to go online and mail-order genetic tests that will predict your genetic risk for common diseases. But because the meaning of the genetic risk factors flagged--how they boost risk for disease--is not yet clear, such tests have received major criticism. In many cases, it's unclear how to use the findings to improve an individual's health.

A new $31 million project aims to address that issue by revisiting large epidemiological studies encompassing tens of thousands of people whose medical status has already been well documented. Scientists will use previously donated DNA samples to identify 100 different variations in the participants, and then sift through data to clarify how they affect health. The results could have a broad impact on public-health recommendations, such as who would benefit most from additional screening for cancer or diabetes. They could also provide new targets for drug development.

The deluge of data linking specific genes to common diseases comes from new tools that allow scientists to quickly scan the genome for up to a million common genetic variations. In the past two years, these so-called genome-wide association studies have identified more than 300 genetic variants that boost risk for illnesses such as diabetes, arthritis, and Crohn's disease, a form of inflammatory-bowel disease. By examining the entire genome without bias, scientists have found genes that they never expected would be involved in a particular disease.

"We're being inundated with [genome-wide association] results, and that's a good thing," says James Evans, a professor of genetics and medicine at the University of North Carolina at Chapel Hill and editor in chief of Genetics in Medicine. "But the next step will be the hardest step, and that is figuring out what it means for human health."

Although genome-wide association studies have shed light on a great number of genetic variations, it's not clear how these genetic changes boost risk for different diseases. For example, a variation that boosts risk for type 2 diabetes might do so by disrupting part of the insulin regulation pathway, or it might act through some less well understood pathway, such as inflammation. Both of those possibilities could point to a specific molecular pathway to target in the development of novel diabetes drugs.

"We want to try to understand more about [these genes], in terms of how they are related to disease and whether they are really causative," says Teri Manolio, director of the Office of Population Genomics at the National Human Genome Research Institute, which is sponsoring the new study.