Previous genetic studies had suggested the group could capture a great deal of the world's genetic diversity by sampling populations of recent European, African, and Asian descent. Still, researchers have begun studies to examine additional populations to see how useful data from the HapMap will be in studying these groups. "The reports I've heard are pretty encouraging," says Donnelly. "For example, the European sample was individuals taken from Utah; but, given the data I've seen, it seems that if you're doing a study in a German population or a British population or a French population, they extrapolate pretty well."

Charles N. Rotimi, who coordinated sample collection for the project among the Yoruba people in Nigeria, emphasizes the importance of studying populations beyond the original four. "We want to make sure that the genetic variations we now have at a very detailed level actually apply to all human populations," says Rotimi, who is director of genetic epidemiology at the National Human Center at Howard University. "The best way to test that is enroll other populations."

Indeed, the U.S. National Human Genome Research Institute has already funded studies of seven other population samples: African ancestry from the southwestern United States; Chinese-Americans from Denver, CO; Gujarati Indian ancestry from Houston, TX; Luhya from Eldoret, Kenya; Maasai from Webuye, Kenya; Mexican origin from Los Angeles, CA; and Tuscans from Sesto, Italy.

By taking advantage of all this data, and then using patients from around the world, geneticists can improve the power of studies to uncover links between gene variations and common diseases, such as cancer, diabetes, and asthma, says Rotimi. The key to uncovering many such links will be international collaborations, researchers say. One simple advantage will be statistical: "By bringing together multiple groups, you immediately increase your power to find these things because you're studying much larger cohorts," says Rotimi.

Geneticists might use studies of multiple populations in other ways, as well, Rotimi notes. One possibility is to begin a gene hunt by studying European populations, which tend to have longer and fewer haplotype blocks, which are easier to test. Then researchers could refine the location by examining older, African groups, which inherit shorter haplotypes, to narrow down the location and perhaps even identify and locate a specific gene.