Critics also see little value in testing healthy people for a wide range of possible diseases. “We don’t take an M.R.I. for everything, and I don’t order every test for every person,” says Harvard geneticist and physician David Altshuler, a key figure in the Human Genome Project. “Those who do are scamming people. It’s the idea that just knowing something is useful—well, maybe, maybe not.”

Part of medicine’s resistance comes from ignorance, counters Lee Hood, a biotech pioneer, M.D., and the president of the Institute for Systems Biology in Seattle. Most physicians have little training in genetic testing, he says, so they have no idea what to tell their patients who hear about tests or want them done. “Health-care professionals need to be pushed to understand this information,” National Human Genome Research Institute director Francis Collins agrees. “Most are not ready for this.”

Ten miles up Highway 101 from 23andMe’s Mountain View office is another online gene-testing startup called Navigenics, which is also in beta mode and plans to launch in early 2008. Focusing on health genetics, it plans to offer consumers a panel of 15 to 20 gene tests, along with detailed information on medical conditions and genetics. “The tests they will run are all clinically validated,” says Brook Byers of Kleiner Perkins Caufield & Byers, the big Silicon Valley venture capital firm, which is investing in the company.

Navigenics C.E.O. Mari Baker, previously the head of Johnson & Johnson’s BabyCenter website and the product manager for Quicken, explains that initially the company will offer tests only for treatable diseases. “Customers will always have the option to exclude certain conditions from their report,” Baker says. One controversial test is for a gene linked to Alzheimer’s disease. “We believe that there are enough promising therapies in development for Alzheimer’s that it makes sense to include it, since recognizing it early is important.”

Navigenics’ market is intended to be people who are healthy and affluent. Customers will be charged between $2,000 and $3,000 to have nearly a million genetic markers tested on a gene chip manufactured by Affymetrix. But Navigenics’ site won’t release all of the data collected by the chip, only the designated panel of gene tests. The company plans to offer information and telephone support from genetic counselors, and a subscription to its service will last a year. “Your DNA will be on file, and we’ll test it against new findings,” says Amy DuRoss, Navigenics’ head of policy and business affairs. Navigenics was co-founded by David Agus, a famed oncologist and geneticist at Cedars-Sinai Medical Center in Los Angeles, and the company has set up a task force to deal with bioethics, patient rights, and data security.

Navigenics is positioned to go after the millions of Americans who are spending money outside of the traditional health-care system to stay healthy, buying vitamins and dietary supplements, gym memberships, and diet and self-help books. “These are people who want to take control of their health,” Baker says. “They will own their own DNA and their results. It’s up to them who they want to share it with; it’s their call. The results will be sent to the patient, not to a doctor.” But Navigenics is also talking to major health-care centers around the country—the Mayo Clinic, the Scripps Research Institute, and the Stanford University Medical Center.

Upstart sites like 23andMe and Navigenics are about to be joined by older genetics companies—including survivors of the late-1990s genomics boom—that run large research labs and develop drugs. One of these is Iceland’s DeCode Genetics. Its DNA studies on the isolated island’s population have led to many of the clinically validated genes—those proved to be strongly associated with a disease—appearing recently in the news. DeCode C.E.O. Kári Stefánsson says that the company plans to launch a direct-to-consumer site next spring. “We have the original data and they don’t,” says Stefánsson about his I.T. competitors.

But once people are done surfing through information on their earwax and fast-twitch muscles, will these companies be able to handle the tough questions that consumers really want answered: Will I get diabetes? Will I contract leukemia? Will I have a heart attack? When?