[[Alkaptonuria]] is a rare inherited genetic disorder in which the body cannot process the [[aminoacid]]s [[phenylalanine]] and [[tyrosine]], which occur in [[protein]]. It is caused by a mutation in the HGD gene for the enzyme homogentisate 1,2-dioxygenase (EC 1.13.11.5); if a person inherits abnormal copies from each parent (it is a [[recessive]] condition), the body accumulates an intermediate substance called [[homogentisic acid]] in the blood and tissues. Homogentisic acid and its oxidized form alkapton are excreted in the urine, giving it an unusually dark color. The accumulating homogentisic acid causes damage to cartilage ([[ochronosis]], leading to osteoarthritis) and heart valves, as well as precipitating as kidney stones and stones in other organs.