HMBS and metabolic disease: Acute intermittent porphyria (AIP) is a rare inherited metabolic disorder characterized by deficient activity of the enzyme hydroxymethylbilane synthase (HMBS), leading to the accumulation of porphyrin precursors, porphobilinogen (PBG) and δ-aminolevulinic acid (d-ALA) in the blood and liver [1].