What are the causes for Acute intermittent porphyria?

Acute intermittent porphyria is caused by a mutation in the gene for porphobilinogen deaminase (PBGD), also known as hydroxymethylbilane synthase (HMBS). However, not all individuals with the abnormal gene exhibit symptoms. The symptoms of acute intermittent porphyria are often triggered by particular medications.

Source: http://www.rightdiagnosis.com