(As of Friday, the California Department of Public Health had not released a list of companies sent letters. Such a list would likely include companies that offer more-traditional genetic tests, such as tests for specific diseases that have a highly heritable component, as well as companies offering tests with largely unsubstantiated scientific claims, such as some nutrigenomics companies. But because of their novelty and a high level of public attention, the most intense debate will likely focus on issues inherent to companies like 23andMe and Navigenics, both of which have confirmed receiving letters.)

23andMe has somewhat skirted the medical issue by emphasizing a more recreational approach to genetic testing, providing ancestry analysis and the ability to compare one's genomes with those of family and friends. The company does, however, offer analysis of customers' genetic risk for various diseases. (23andMe declined to give specifics on how it will respond to the health department's letter.)

"On the one hand, arguing that it's not a medical diagnosis helps to enforce the limitations of this information," says Amy McGuire, a medical ethicist at Baylor College of Medicine, in Houston. "Most physicians would not in the context of medical practice order the testing. But regardless of this, most consumers will misinterpret it--they'll think of the results as medical information and treat it that way."

Navigenics, in contrast, focuses on medical risk and offers genetic counseling as part of its testing service. In a statement released last week, Navigenics emphasized that the company's tests are ordered and reviewed by a California-licensed physician. Whether or not that will satisfy California's public-health officials--and the broader community--remains to be seen.

"If you look at a Navigenics report, every page says that this is not intended to be medical or diagnostic information," says James Evans, a clinical geneticist at the University of North Carolina at Chapel Hill and editor in chief of the journal Genetics in Medicine. "Yet their entire marketing strategy is clearly based on the idea that this is useful medical information." Navigenics had not returned calls for comment at press time.

All three of the genome-wide screening companies make a significant effort to put the genetic-risk information in context. But some critics say that this may not be enough. "I'd be more interested in seeing this argued as a false-claims issue," says Arthur Caplan, director of the Center for Bioethics at the University of Pennsylvania, in Philadelphia. "It's the provider's challenge to make sure you get an informed consumer when you are talking about genetic information."