Types of sialyltransferases and their main acceptor substrates. STs can link sialic acid (Sia) to terminal galactose (Gal) via α2,6 or α2,3-linkage or to galactosamine or N-acetyl-galactosamine (GalNAc) via α2,6-linkage. Also, Sia can be linked to the C8 position of another Sia residue forming polysialic acids. CMP: cytidine monophosphate.