The recent breach of 23andMe user accounts shows a simple yet powerful truth about data security: Don’t reuse passwords, people.

The DNA test-kit company on Monday reported a hacker accessed 14,000 accounts because of password reuse, exposing information belonging to approximately 6.9 million people. The 23andMe computer network wasn’t breached and wasn’t the source of these compromised credentials, a company spokesman said in a statement. The company first disclosed the incident in October and has been investigating since then.