Carbamoyl phosphate synthetase I deficiency is an inherited disorder that causes ammonia to accumulate in the blood (hyperammonemia). Ammonia, which is formed when proteins are broken down in the body, is toxic if the levels become too high. The brain is especially sensitive to the effects of excess ammonia.

In the first few days of life, infants with carbamoyl phosphate synthetase I deficiency typically exhibit the effects of hyperammonemia, which may include unusual sleepiness, poorly regulated breathing rate or body temperature, unwillingness to feed, vomiting after feeding, unusual body movements, seizures, or coma. Affected individuals who survive the newborn period may experience recurrence of these symptoms if diet is not carefully managed or if they experience infections or other stressors. They may also have delayed development and intellectual disability.

In some people with carbamoyl phosphate synthetase I deficiency, signs and symptoms may be less severe and appear later in life.

Carbamoyl phosphate synthetase I deficiency is a rare disorder; its overall incidence is unknown. Researchers in Japan have estimated that it occurs in 1 in 800,000 newborns in that country.

Mutations in the CPS1 gene cause carbamoyl phosphate synthetase I deficiency. The CPS1 gene provides instructions for making the enzyme carbamoyl phosphate synthetase I. This enzyme participates in the urea cycle, which is a sequence of biochemical reactions that occurs in liver cells. The urea cycle processes excess nitrogen, generated when protein is broken down by the body, to make a compound called urea that is excreted by the kidneys. The specific role of the carbamoyl phosphate synthetase I enzyme is to control the first step of the urea cycle, a reaction in which excess nitrogen compounds are incorporated into the cycle to be processed.

Carbamoyl phosphate synthetase I deficiency belongs to a class of genetic diseases called urea cycle disorders. In this condition, the carbamoyl phosphate synthetase I enzyme is at low levels (deficient) or absent, and the urea cycle cannot proceed normally. As a result, nitrogen accumulates in the bloodstream in the form of toxic ammonia instead of being converted to less toxic urea and excreted. Ammonia is especially damaging to the brain, and excess ammonia causes neurological problems and other signs and symptoms of carbamoyl phosphate synthetase I deficiency.

Genetic Testing Information

• Genetic Testing Registry: Congenital hyperammonemia, type I

Genetic and Rare Diseases Information Center

• Carbamoyl-phosphate synthetase 1 deficiency

Patient Support and Advocacy Resources

• National Organization for Rare Disorders (NORD)

Clinical Trials

• ClinicalTrials.gov

Catalog of Genes and Diseases from OMIM

• CARBAMOYL PHOSPHATE SYNTHETASE I DEFICIENCY, HYPERAMMONEMIA DUE TO

Scientific Articles on PubMed

• PubMed

• Aoshima T, Kajita M, Sekido Y, Mimura S, Itakura A, Yasuda I, Saheki T, Watanabe K, Shimokata K, Niwa T. Carbamoyl phosphate synthetase I deficiency: molecular genetic findings and prenatal diagnosis. Prenat Diagn. 2001 Aug;21(8):634-7. doi: 10.1002/pd.123. Citation on PubMed
• Endo F, Matsuura T, Yanagita K, Matsuda I. Clinical manifestations of inborn errors of the urea cycle and related metabolic disorders during childhood. J Nutr. 2004 Jun;134(6 Suppl):1605S-1609S; discussion 1630S-1632S, 1667S-1672S. doi: 10.1093/jn/134.6.1605S. Citation on PubMed
• Finckh U, Kohlschutter A, Schafer H, Sperhake K, Colombo JP, Gal A. Prenatal diagnosis of carbamoyl phosphate synthetase I deficiency by identification of a missense mutation in CPS1. Hum Mutat. 1998;12(3):206-11. doi: 10.1002/(SICI)1098-1004(1998)12:33.0.CO;2-E. Citation on PubMed
• Haberle J, Schmidt E, Pauli S, Rapp B, Christensen E, Wermuth B, Koch HG. Gene structure of human carbamylphosphate synthetase 1 and novel mutations in patients with neonatal onset. Hum Mutat. 2003 Apr;21(4):444. doi: 10.1002/humu.9118. Citation on PubMed
• Rapp B, Haberle J, Linnebank M, Wermuth B, Marquardt T, Harms E, Koch HG. Genetic analysis of carbamoylphosphate synthetase I and ornithine transcarbamylase deficiency using fibroblasts. Eur J Pediatr. 2001 May;160(5):283-7. doi: 10.1007/s004310100725. Citation on PubMed
• Simpson KL, MacLeod EL, Kakajiwala A, Gropman AL, Ah Mew N. Urea Cycle Disorders Overview. 2003 Apr 29 [updated 2025 Jul 3]. In: Adam MP, Feldman J, Mirzaa GM, Pagon RA, Wallace SE, Amemiya A, editors. GeneReviews(R) [Internet]. Seattle (WA): University of Washington, Seattle; 1993-2025. Available from http://www.ncbi.nlm.nih.gov/books/NBK1217/ Citation on PubMed
• Wakutani Y, Nakayasu H, Takeshima T, Adachi M, Kawataki M, Kihira K, Sawada H, Bonno M, Yamamoto H, Nakashima K. Mutational analysis of carbamoylphosphate synthetase I deficiency in three Japanese patients. J Inherit Metab Dis. 2004;27(6):787-8. doi: 10.1023/b:boli.0000045842.59768.ea. Citation on PubMed