Three gene mutations may be associated with cases of attention deficit hyperactivity disorder (ADHD), say German researchers.

They studied 329 families in which one child had ADHD, finding that about 70 percent of the children had a combination of three mutations in the gene for a dopamine transporter linked to hyperactivity, Agence France Presse reported.

"People who have this combination in both copies of the gene have a 2.5 increased ADHD risk. People with only one copy of this variant still have almost twice the risk," team leader Dr. Johannes Hebebrand of the University of Duisburg-Essen said in a prepared statement.

"Of course, this doesn't mean that everyone who has the genetic variants will automatically get the disease," he added.

Hebebrand said this is the strongest genetic evidence found to date linking the dopamine transporter to hyperactivity, AFP reported.