The enzyme aconitate decarboxylase (i.e., ACOD1, also termed cis-aconitate decarboxylase, immune-responsive gene 1, immune response gene 1, and IRK1 ) is a protein enzyme that in humans is encoded by the gene located at position 22.3 on the long arm (i.e., p-arm) of chromosome 13. ACOD1 catalyzes the following reversible (i.e., runs in both directions, as indicated by ) decarboxylation chemical reaction: Hence, ACOD1 converts cis-aconitate into two products, itaconate and CO2 or itaconate and CO2 into one product, aconitate. ACOD1 belongs to the family of lyases, specifically the carboxy-lyases, which cleave carbon-carbon bonds. The systematic name of this enzyme class is cis-aconitate carboxy-lyase (itaconate-forming). Other names once in common use for this enzyme class include CAD and cis-aconitate carboxy-lyase. ACOD1 participates in c5-branched dibasic acid metabolism. Ustilago maydis (a species of Ustilago fungi) converts cis-aconitate to its thermodynamically favored product, trans-aconitate, by the enzyme aconitate delta-isomerase (i.e., Adi1). The trans-aconitate product is decarboxylated to itaconate by trans-aconitate decarboxylase (i.e., Tad1). This Adi followed by Tad 1 enzymatic metabolic pathway is: Trans-aconitate decarboxylase does not metabolize cis-aconitate to itaconate. (The genes for aconitate delta-isomerase and trans-aconitate decarboxylase have been reported in several types of fungi hut not in other organisms, including humand, and are classified as provisional, i.e., accepted provisional to further studies.)