Alkaptonuria (AKU, OMIM#203500), results from mutations in the homogentisate 1,2 dioxygenase gene (HGD, EC 1.13.11.5),1 increasing circulating homogentisic acid (HGA) and urinary HGA, an intermediary metabolite in the catabolism of the amino acids phenylalanine and tyrosine. Here, HGD is linked to alkaptonuria.