Urea cycle disorder: one of a number of inherited medical conditions that arise due to a deficiency in one of six enzymes involved in the urea cycle. The urea cycle is a series of biochemical reactions that convert nitrogen, a waste product of protein metabolism, to a compound called urea. Urea is removed from the blood by the kidneys and excreted in the urine. In urea cycle disorders, nitrogen levels builds up in the blood as ammonia, which is a toxic substance. The medical term for this build-up of ammonia is hyperammonemia (elevated blood ammonia). Ammonia reaches the brain through circulation of the blood, where it can cause irreversible brain damage, coma and/or death. The onset and severity of symptoms of the urea cycle disorders is highly variable and depends on the particular enzyme that is deficient and the level of enzyme function that remains in a given individual.