**Alkaptonuria happens when you inherit changes in both copies of the HGD gene, leaving your body unable to properly break down homogentisic acid.**
This acid forms as your body breaks down the protein building blocks tyrosine and phenylalanine. You usually inherit one altered gene copy from each parent, who can carry it without symptoms. The acid can make your urine darken after exposure to air and gradually cause pigment deposits and joint problems. Your eating habits do not cause this inherited condition. If you have symptoms or a family history, discuss testing with your clinician and get guidance before restricting protein.
What causes it
Common questions
Why does Alkaptonuria happen?
Alkaptonuria happens when you inherit disease-causing changes in both copies of the HGD gene, usually one from each parent. These changes reduce an enzyme's ability to process homogentisic acid, a substance made when your body breaks down tyrosine and phenylalanine. The acid builds up, causing urine to darken after exposure to air and pigment to collect in cartilage over time. Your parents can carry the gene change without symptoms. If your urine turns brown or black, see a clinician for evaluation and urine testing; color alone cannot establish the diagnosis.