Metabolic activation of sialic acid for oligosaccharide biosynthesis involves formation of CMP-sialic acid.

CTP + Sialic acid <=> CMP-Sialic Acid + PPi

This intermediate is unusual for two reasons. First, other intermediates in oligosaccharide biosynthesis involve nucleoside diphosphates (such as UDP-Glucose). Second, CMP-sialic acid is synthesized in the nucleus of animal cells, whereas all other known nucleotide-linked sugars are synthesized in the cytosol.