Mutations in the third enzyme of this synthesis, HMBS [hydroxymethylbilane synthase; also known as PBG (porphobilinogen) deaminase, EC 2.5.1.61] are associated with the most frequently occurring acute porphyria, AIP (acute intermittent porphyria), an autosomal dominant inherited disease. The gene discussed is HMBS; the disease is acute intermittent porphyria.