Acute intermittent porphyria (AIP; MIM: 176000) is caused by deficiency in the heme biosynthetic enzyme hydroxymethylbilane synthase (HMBS; MIM: 609806, EC 2.5.1.61; also known as porphobilinogen deaminase [PBGD]).1 The gene discussed is HMBS; the disease is autoimmune pancreatitis.