HMBS and Acute hepatic porphyria: Acute intermittent porphyria (AIP), the most common form of acute hepatic porphyria, is an autosomal dominant disorder caused by a partial deficiency in the activity of the third enzyme in the heme biosynthesis pathway, hydroxymethylbilane synthase (HMBS), also known as porphobilinogen deaminase (PBGD) (Puy et al., 2010).