Alkaptonuria (AKU; OMIM #203500) is an autosomal recessive, multisystem disease caused by mutations in the homogentisate 1,2‐dioxygenase (HGD) enzyme (EC 1.13.11.5).1 HGD deficiency leads to homogentisic acid (HGA) accumulation (Figure 1A) in the blood and tissues. The gene discussed is HGD; the disease is alkaptonuria.