OTC and glycogen storage disease VI: Urea cycle disorders (UCDs) are a group of rare inborn diseases caused by either a deficiency in one of the six enzymes—carbamoylphosphate synthetase I (CPS1), ornithine transcarbamylase (OTC), argininosuccinic acid synthetase (ASS1), argininosuccinic acid lyase (ASL), arginase (ARG1), N-acetyl glutamate synthetase (NAGS)—or one of the two transporters—ornithine translocase (ORNT1) and citrin (aspartate/glutamate carrier)—involved in the urea cycle (Figure 1) [1].