Alkaptonuria (black urine disease, black bone disease, or alcaptonuria) is a rare inherited genetic disorder in which the body cannot process the amino acidsphenylalanine and tyrosine, which occur in protein. It is caused by a mutation in the HGDgene for the enzymehomogentisate 1,2-dioxygenase ; 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 oxidated formalkapton 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. Symptoms usually develop in people over thirty years old, although the dark discoloration of the urine is present from birth.