N-Acetylneuraminic acid (Neu5Ac), N-glycolylneuraminic acid (Neu5Gc) andN-acetyl-9-O-acetylneuraminic acid (Neu5,9Ac2) are the three most frequently
occurring members of the Sia family. Only Neu5Ac is
ubiquitous, while the others are not found in all species.
The best investigated example next to Neu5Ac is
Neu5Gc, which occurs frequently in the animal kingdom, but not in healthy human tissues—only in some tumors in minute quantities (Malykh et al., 2001a)—and
has not been detected in bacteria.