Acute intermittent porphyria (AIP), HCP, and VP are inherited in an autosomal dominant way and result from a deficiency of one of the enzymes, hydroxymethylbilane synthase (HMBS), coproporphyrinogen oxidase (CPOX), and protoporphyrinogen oxidase (PPOX), respectively (Figure 1). The gene discussed is PPOX; the disease is acute intermittent porphyria.