Porphyrias are inherited defects in the biosynthesis of heme. Acute intermittent porphyria (https://omim.org/entry/176000), the most common form of porphyria, is an autosomal dominant disorder characterized by recurrent attacks of abdominal pain, gastrointestinal dysfunction, and neurologic disturbances. In the classic form of AIP, both the ubiquitous ‘nonerythroid’ housekeeping HMBS isoform and the ‘erythroid’ HMBS isoform are deficient. However, about 5% of families have the ‘nonerythroid variant’ of AIP, with a defect only in the ubiquitous nonerythroid HMBS isoform and normal levels of the erythroid HMBS isoform. Clinical characteristics in the 2 forms are identical; diagnostic methods based on the level of enzyme in erythrocytes is ineffective (Puy et al., 1998Petrides, 1998Whatley et al., 2000).

There are several other forms of porphyria: see porphyria cutanea tarda (176100), variegata porphyria (176200), coproporphyria (121300), acute hepatic porphyria (125270), and congenital erythropoietic porphyria (263700).