Genetic deficiency of homogentisate 1,2 dioxygenase (HGD, EC 1.13.11.5) activity, inherited as an autosomal recessive trait, causes alkaptonuria (AKU) (OMIM 203500), resulting in accumulation of homogentisic acid (HGA), leading to increased serum concentrations and increased excretion in the urine from birth (Figure S1).1 The gene discussed is HGD; the disease is alkaptonuria.