Two types of Andersen–Tawil syndrome have been described, distinguished by the genetic abnormality that is detected. Type 1 Andersen–Tawil, accounting for about 60% of cases, is caused by mutations in the "KCNJ2" gene. In type 2 Andersen–Tawil, accounting for about 40% of cases, a "KCNJ2" mutation is not identified. Mutations in a related gene encoding a similar potassium ion channel, "KCNJ5", have been identified in some of those with type 2 Andersen–Tawil, but in many cases a genetic mutation is not found.