CMP-Sialic acid is a nucleotide sugar consisting of cytidine 5-monophospho-N-acetylneuraminic acid, a phosphate-bound form of sialic acid. Nucleotide sugars are the activated forms of monosaccharides required for cellular metabolism and biosynthesis reactions. CMP-Neu5Ac serves as a donor substrate for sialyltransferases in the biosynthesis of sialic acid-containing glycans.