ASS1 and urea cycle disorder: Citrullinemia is a rare autosomal recessive urea cycle disorder caused by mutations in either ASS1 (type 1), encoding argininosuccinate synthetase which is an urea cycle enzyme, or SLC25A13 (type 2, also called citrin deficiency), encoding the aspartate-glutamate carrier (citrin) which results in insufficient aspartate supply to the urea cycle (47).