A urea cycle disorder (UCD) is an inherited disease that affects how the body removes the waste that is made from breaking down protein. Everyone needs protein, which is found in foods like dairy products, meat and fish. When a person eats food that contains protein, the body breaks it down into amino acids (the building blocks of protein that are used by the body for growth and tissue repair) and uses only what it needs. It changes the rest into nitrogen, which must then be removed by the body.

In a healthy person, the liver supplies several enzymes to change nitrogen into urea, which is then removed from the body in urine. This entire process is called the urea cycle.

The liver in a person with urea cycle disorder is missing an enzyme necessary to convert nitrogen into urea. As a result, ammonia, a highly toxic substance, builds up in the bloodstream and is not removed from the body. Untreated, the high amounts of ammonia can cause brain damage, coma and eventually death.