There are six enzyme disorders of the urea cycle, collectively known as inborn errors of urea synthesis, or urea cycle enzyme defects. Each is referred to by the initials of the missing enzyme.

Additionally, there are three transporter defects:

	Mitochondrial ornithine carrier (Hyperornithinemia-Hyperammonemia-Homocitrullinemia or HHH syndrome)
	Mitochondrial aspartate/glutamate carrier (Citrullinemia Type II)
	Dibasic amino acid carrier (Hyperdibasic Amino Aciduria or Lysinuric Protein Intolerance)

Neonatal onset disorders represent severe enzyme deficiencies or complete absence of enzyme function.  Individuals with childhood or adult onset disease have partial enzyme deficiency.  The percentage, or amount of enzyme function, varies widely between individuals with partial enzyme deficiencies.  All of these disorders are transmitted genetically as autosomal recessive genes - each parent contributes a defective gene to the child - except for Ornithine Transcarbamylase Deficiency.  OTC deficiency is acquired in one of three ways: as an X-linked trait from the mother, who may be an undiagnosed carrier; in some cases of female children, the disorder can also be inherited from the defect on the father's X-chromosome; and finally, OTC deficiency may be acquired as a "new" spontaneous mutation occurring in the fetus. Recent research has shown that some female carriers of the disease may become symptomatic with the disorder later in life, suffering high ammonia levels and experiencing classic symptoms. Several undiagnosed women have died during childbirth as a result of high ammonia levels and on autopsy were determined to have been unknown symptomatic carriers of the disorder.