Cytidine Monophosphate N-Acetylneuraminic Acid Synthetase1 (CMAS) transfers cytidine 5-monophosphate from a CTP donor to N-acetylneuraminate (Neu5Ac) to form CMP-Neu5Ac. CMAS is ubiquitously expressed and localizes to the nucleus in mammalian cells. The active form of the enzyme is a homotetramer (a dimer of dimers) (Lawrence et al. 2001, Huizing 2005). CMP-Neu5Ac is the donor substrate for sialyltransferases.