HGD and alkaptonuria: Alkaptonuria (AKU, OMIM: 203500) is a rare bi-allelic autosomal recessive disorder of the tyrosine metabolic pathway, occurring 1 in 100,000–250,000 of the general population (Phornphutkul et al. 2002) and arises from a congenital deficiency in the enzyme homogentisate-1,2-dioxygenase (HGD, E.C.1.12.11.5) (Fig. 1).