Catalyzes the activation of N-acetylneuraminic acid (NeuNAc) to cytidine 5'-monophosphate N-acetylneuraminic acid (CMP-NeuNAc), a substrate required for the addition of sialic acid. an N-acylneuraminate + CTP = a CMP-N-acyl-beta-neuraminate + diphosphate Amino-sugar metabolism; N-acetylneuraminate metabolism. Homotetramer; the active enzyme is formed by a dimer of dimers. Expressed in testis, ovary and liver. Belongs to the CMP-NeuNAc synthase family.