Alpha-N-acetylgalactosaminide α-2,6-sialyltransferase 1 catalyzes the transfer of sialic acid N-acetylneuraminic acid (Neu5Ac) from cytidine 5'-monophosphono-N-acetylneuraminic acid (CMP-Neu5Ac) to O-linked GalNAc residues in an α-2,6 linkage. ST6GalNAcI belongs to a large family of Golgi-membrane-bound sialyltransferases that is responsible for the biosynthesis of sialyated structures, and from which approximately 20 have been cloned to date. The cancer associated sialyl-Tn (sTn) angtigen is formed by ST6GalNAcI, through the transfer of Neu5Ac to GalNAc resiudes on mucins. Addition of a sialic acid to a glyco-protein effectively caps the glycan-chain, preventing further additions. The presence of sialic acid residues extends th