HGD and alkaptonuria: Alkaptonuria (AKU; Online Mendelian Inheritance in Man (OMIM): 203500) is a rare genetic autosomal recessive condition caused by mutations within a gene coding for homogentisate 1,2 dioxygenase (HGD, Enzyme Commission (EC): 1.13.11.5) that plays a role in the tyrosine catabolism pathway by converting homogentisic acid (HGA) into maleylacetoacetic acid; this mutation will result in accumulation of HGA in the body.