Consumer genome testing companies Navigenics Inc. and 23andMe, co-owned by the wife of Google co-founder Sergey Brin, refused orders of the California Health Department to cease accepting clients by continuing operations on Wednesday, a day after the orders were posted on the web. They aver that that their operations already follow the law.

The two Bay Area companies are among 13 genetic testing outfits that were ordered to cease and desist this month by the state Department of Public Health. Other prominent names include Knome Inc. in Cambridge and deCODEme Genetics of Iceland.

The government agency warned the companies that they must obtain a clinical laboratory license before conducting medical tests for Californians. In addition, the agency said the companies couldn't offer laboratory tests directly to California consumers without a doctor's order.

"The intent of California's clinical laboratory laws is to protect the public by ensuring that results are accurate and reliable," the state Public Health Department said in a statement issued Tuesday. "The physician assists the patient by ensuring that appropriate tests are ordered and interpreting the test results."

New York also sent letters to a half-dozen genetic testing firms a few months ago. But federal agencies are still discussing whether to take any action, said Gail Javitt, law and policy director of the Genetics and Public Policy Center at Johns Hopkins University.

Navigenics of Redwood City offers clients a $2,500 scan of their DNA for signs that they might be at higher risk for diseases such as cancer, diabetes and other ailments that may be preventable by improving health habits.

Mountain View-based 23andMe is co-owned by the wife of Google co-founder Sergey Brin and was partially funded by his company. It offers a DNA scan for $999.

Knome has gone further than most companies by offering to sequence an entire genome - rather than just key strands of DNA - for $350,000. In January, the company unveiled plans to accept its first 20 patients. It has teamed up with the Beijing Genomics Institute, which will do the actual DNA sequencing while Knome focuses on signing up customers and explaining what the sequence means.

Navigenics said it contracts with doctors to review online requests for the genome scans. The doctor, who does not know the client's name, orders the test from Navigenics' partner in the enterprise, Affymetrix Inc. of Santa Clara. Navigenics CEO Baker said Affymetrix, a leading genetic analysis company, holds all necessary state and federal licenses.

"We invested a significant amount of time and energy up front for one and a half years before the launch (of the business in April) to understand the regulatory environment across the country and make sure we were in compliance," said Navigenics chief executive Mari Baker. "We were surprised to get the letter."

23andMe also declared their compliance with state regulations, but added, ''However, we would like to have continued discussions with the department regarding the appropriate regulation of this unique industry.''

In a statement, Knome chief executive and cofounder Jorge Conde said clients already have a doctor involved in the process, but he did not address licensing.

"While we have not yet officially responded to the letter," Conde said, "we will seek an ongoing dialogue with the California Department of Public Health" about state regulations.

The personal genome services stay away from diagnosing specific ailments, instead focusing on patterns of genetic variation that may indicate health risks such as heart disease, where diet, exercise and other factors are also important causes.

Customers are invited to order the DNA scans online and send in their own samples of saliva by mail. The companies have varying styles of conveying the meaning of the tests to customers, including Web-based reports. They insist they are not practicing medicine, but providing an educational service.

In addition, 23andMe adds an element of entertainment with a social-networking feature that allows clients to share DNA data with friends and family members.