Acute intermittent porphyria (AIP), an autosomal dominant inborn error of heme biosynthesis, is due to pathogenic mutations in the hydroxymethylbilane synthase (Hmbs) gene which reduce the activity of its encoded enzyme (HMBS or porphobilinogen deaminase) by approximately 50% [1, 2]. The gene discussed is HMBS; the disease is autoimmune pancreatitis.