Sialic acids, commonly present as terminal carbohydrates on glycoconjugates, are essential for a variety of cellular functions including cell adhesion and signal recognition, as well as the formation and progression of tumors. 3-Deoxy-D-glycero-D-galacto-2-nonulosonic acid is a deaminated sialic acid that was first identified at the nonreducing ends of oligosialyl chains in rainbow trout egg glycoprotein.[1] It is thought to be a precursor for the biosynthesis of other members of the sialic acid family.[2] 3-Deoxy-D-glycero-D-galacto-2-nonulosonic acid can be used to analyze nonulosonic acid residues in polysialoglycoproteins.[1]

Reference:
[1]. Nadano, D., Iwasaki, M., Endo, S., et al. A naturally occurring deaminated neuraminic acid, 3-deoxy-D-glycero-D-galacto-nonulosonic acid (KDN). Its unique occurrence at the nonreducing ends of oligosialyl chains in polysialoglycoprotein of rainbow trout eggs. The Journal of Biological Chemisty 261(25), 11550-11557 (1986).
[2]. Münster-Kühnel, A.K., Tiralongo, J., Krapp, S., et al. Structure and function of vertebrate CMP-sialic acid synthetases. Glycobiology 14(10), 43R-51R (2004).