HMBS is the third enzyme in the heme synthesis pathway that catalyzes 4 porphobilinogen (PBG) into the hydroxymethylbilane.[1] Mutation of the HMBS gene leads to HMBS deficiency and accumulation of porphyrin precursors including δ-amino levulinic acid (ALA) and PBG. The gene discussed is HMBS; the disease is acute intermittent porphyria.