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Urea Cycle Disorders

When our bodies metabolize protein, one waste product is nitrogen. Normally this excess nitrogen is converted to urea and is excreted in urine. When a person has a urea cycle disorder their body is missing enzymes that remove nitrogen (as urea) from the bloodstream. When nitrogen is not excreted, it can build up in the bloodstream in the form of ammonia. This buildup of ammonia can be highly toxic if the ammonia is not removed.

There are six types of urea cycle disorders.

NAGS deficiency (N-acetyl glutamate synthetase)
CPS 1 deficiency (carbamoyl phosphate synthetase)
OTC deficiency (ornithine transcarbamylase)
CTLN1 deficiency, also known as citrullinemia type 1, ASD or ASL (argininosuccinate synthetase)
ALD deficiency, also known as argininosuccinic aciduria, ASA, ASL or AL (arginino succinase)
ARG deficiency, also called hyperargininemia (arginase)

There are also two enzyme transporter deficiencies, which are often considered UCDs:

HHH Syndrome, also called ORNT deficiency (ornithine translocase)
CTLN2 deficiency, also known as citrullinemia type II (citrin)

Here are health guides on urea cycle disorders that we’ve created for patients, families, and educators —

An Educator’s Guide to Urea Cycle Disorders: for educators with students who have urea cycle disorders

Transition Toolkit: for teens with metabolic disorders transitioning to adulthood


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