The synthesis of carbamoyl phosphate from glutamine, bicarbonate, and ATP is catalyzed by the carbamoyl-phosphate synthase (glutamine-hydrolyzing) activity of cytosolic trifunctional CAD (carbamoyl-phosphate synthetase 2, aspartate transcarbamylase, and dihydroorotase) protein (Ito and Uchino 1973; Iwahana et al. 1996). The purified human protein is active in several different oligomerization states, as is its Syrian hamster homologue. The most abundant form of the latter is a hexamer, and the active human protein is annotated as a hexamer by inference (Ito and Uchino 1973; Lee et al. 1985).