Alkaptonuria (black urine disease or alcaptonuria) is a rare inherited genetic disorder of tyrosine metabolism. This is an autosomal recessive condition that is due to a defect in the enzyme homogentisate 1,2-dioxygenase (EC 1.13.11.5), which participates in the degradation of tyrosine. As a result, a toxic tyrosine byproduct called homogentisic acid (or alkapton) accumulates in the blood, and is excreted in urine in large amounts(hence -uria). Excessive homogentisic acid causes damage to cartilage (ochronosis, leading to osteoarthritis) and heart valves as well as precipitating as kidney stones. Treatment with nitisinone, which suppresses homogentisic acid production, is being studied. Alkaptonuria is more common in Slovakia and the Dominican Republic than in other countries.