The biosynthesis of disaccharides oligosaccharides and polysaccharides involves the action of hundreds of different glycosyltransferases. These enzymes catalyse the transfer of sugar moieties from activated donor molecules to specific acceptor molecules forming glycosidic bonds. A classification of glycosyltransferases using nucleotide diphospho-sugar nucleotide monophospho-sugar and sugar phosphates () and related proteins into distinct sequence based families has been described . This classification is available on the CAZy (CArbohydrate-Active EnZymes) web site. The same three-dimensional fold is expected to occur within each of the families. Because 3-D structures are better conserved than sequences several of the families defined on the basis of sequence similarities may have similar 3-D structures and therefore form 'clans'.Glycosyltransferase family 29 () comprises enzymes with a number of known activities; sialyltransferase () beta-galactosamide alpha-2,6-sialyltransferase () alpha-N-acetylgalactosaminide alpha-2,6-sialyltransferase () beta-galactoside alpha-2,3-sialyltransferase () N-acetyllactosaminide alpha-2,3-sialyltransferase () alpha-N-acetyl-neuraminide alpha-2,8-sialyltransferase (); lactosylceramide alpha-2,3-sialyltransferase (). These enzymes use a nucleotide monophosphosugar as the donor (CMP-NeuA) instead of a nucleotide diphosphosugar. Sialyltransferase may be responsible for the synthesis of the sequence NEUAC-Alpha-2,3-GAL-Beta-1,3-GALNAC- found on sugar chains O-linked to thr or ser and also as a terminal sequenec on certain gagnliosides. These enzymes catalyse sialyltransfer reactions during glycosylation and are type II membrane proteins.