The protein encoded by this gene is a type II membrane protein that catalyzes the transfer of sialic acid from CMP-sialic acid to galactose-containing substrates. The encoded protein prefers glycoproteins rather than glycolipids as substrates and shows restricted substrate specificity, utilizing only the trisaccharide sequence Neu5Ac-alpha-2,3-Gal-beta-1,3-GalNAc. In addition, it is involved in the synthesis of ganglioside GD1A from GM1B. The encoded protein is normally found in the Golgi apparatus but can be proteolytically processed to a soluble form. This protein is a member of glycosyltransferase family 29. Transcript variants encoding different isoforms have been found for this gene. [provided by RefSeq, Jul 2008].