In an internal memo, Pentagon leadership has urged military personnel not to take mail-in DNA tests, warning that they create security risks, are unreliable and could negatively affect service members’ careers.

The letter, which was reported by Yahoo News, was sent on Friday. It does not name any particular DNA testing companies, but counsels broadly against buying ancestry and health tests promoted with military discounts and other military incentives.

Cmdr. Sean Robertson, a Pentagon spokesman, confirmed that the memo had been sent.

“We want to ensure all service members are aware of the risks of Direct to Consumer (DTC) genetic testing,” he told The New York Times over email.

Over the past decade, millions of Americans have purchased DNA tests through companies such as 23andMe and Ancestry with the hopes of connecting with relatives, finding out more about their family origins and learning about how their DNA could affect their chances of developing certain health conditions. In recent years, the tests have become popular holiday gifts.