There are three main types of HAE. Types I and II are caused by a mutation in the "SERPING1" gene, which makes the C1 inhibitor protein, while type III is often due to a mutation in the "F12" (factor XII) gene. The result is increased levels of bradykinin, which promotes swelling. The condition may be inherited from a person's parents in an autosomal dominant manner or occur as a new mutation. Triggers for an attack may include minor trauma or stress, but attacks often occur without any obvious preceding event. Diagnosis of types I and II is based on measurement of C4 and C1-inhibitor levels.