A urea cycle disorder is a genetic disorder caused by a deficiency of one of the enzymes in the urea cycle which is responsible for removing ammonia from the blood stream. The urea cycle involves a series of biochemical steps in which nitrogen, a waste product of protein metabolism, is removed from the blood and converted to urea. Normally, the urea is transferred into the urine and removed from the body. In urea cycle disorders, the nitrogen accumulates in the form of ammonia, a highly toxic substance, and is not removed from the body resulting in hyperammonemia. Ammonia then reaches the brain through the blood, where it causes irreversible brain damage, coma and/or death.

Urea cycle disorders are included in the category of inborn errors of metabolism. There is no cure. Inborn errors of metabolism represent a substantial cause of brain damage and death among newborns and infants. Because many cases of urea cycle disorders remain undiagnosed and/or infants born with the disorders die without a definitive diagnosis, the exact incidence of these cases is unknown and underestimated. It is believed that up to 20% of Sudden Infant Death Syndrome cases may be attributed to an undiagnosed inborn error of metabolism such a urea cycle disorder. In April 2000, research experts at the Urea Cycle Consensus Conference estimated the incidence of the disorders at 1 in 10,000 births. This represents a significant increase in case diagnosis in the last few years.  Research studies have now been initiated to more accurately determine the incidence and prevalence of UCDs.