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. Also has activity towards N-glycolylneuraminic acid (Neu5Gc). Has weak activity towards 2-keto-3-deoxy-D-glycero-D-galacto-nononic acid (KDN). an N-acylneuraminate + CTP = a CMP-N-acyl-beta-neuraminate + diphosphate Amino-sugar metabolism; N-acetylneuraminate metabolism. Homotetramer. Detected in the axial mesoderm at 9 hours post-fertilization (hpf). Expressed in the central nervous system, somites, notochord and developing pronephric duct at 18 hpf. At later stages (48-72 hpf) expression decreases in the trunk but persists in the central nervous system, and is also found in liver and kidney primordia. Belongs to the CMP-NeuNAc synthase family. Truncated C-terminus. Truncated N-terminus. Sequence of unknown origin at N terminus.