Carbamoyl phosphate synthase (CPS) deficit, ornithine transcarbamylase (OTC) deficiency, argininosuccinate synthetase deficiency (citrullinemia), argininosuccinate lyase deficiency (argininosuccinic aciduria), and arginase deficiency are all examples of primary UCDs (argininemia). Urea Cycle Disorders (UCDs) are a group of inborn errors of metabolism that affect the function of any number of proteins involved in the urea cycle. The cycle is responsible for removing nitrogen from protein into an organic form that can be expelled from the body in urine. This process requires nine different enzymes. When any of these enzymes is unable to function properly, then nitrogen cannot be removed from protein and it builds up in the blood and tissues of affected people. This condition leads to tissue damage and may cause death if not treated with appropriate medications. People who have UCDs cannot make some or all of the enzymes needed for the urea cycle. This means that they must get their own proteins building blocks from food which then need to be processed by other cells in the body. Because these enzymes are so important for survival, there are many studies trying to find ways to treat people with UCDs. Some treatments aim to replace the missing enzyme with either purified protein or DNA containing the gene for the enzyme.