In alkaptonuria (AKU) (OMIM#203500), a disorder of the tyrosine (TYR) pathway, the lack of homogentisate 1,2 dioxygenase (HGD) (EC.1.13.11.5) activity results in failure to convert homogentisic acid (HGA) to maleylacetoacetic acid, thus leading to accumulation of HGA and the damaging effects of AKU1,2. The gene discussed is HGD; the disease is alkaptonuria.