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. Has some activity toward NeuNAc, N-glycolylneuraminic acid (Neu5Gc) or 2-keto-3-deoxy-D-glycero-D-galacto-nononic acid (KDN) (By similarity). 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. Liver. The BC2 (basic cluster 2) motif is necessary and sufficient for the nuclear localization and contains the catalytic active site. The localization in the nucleus is however not required for the enzyme activity (By similarity). Belongs to the CMP-NeuNAc synthase family.