ST3 beta -Gal alpha -2,3-Sialyltransferase 1 (ST3GAL1, also known as SIAT4 and SIAT4A) is a 45 kDa member of glycosyltransferase family 29 that catalyzes the transfer of sialic acid from CMP-sialic acid to galactose-containing substrates. The full length ST3GAL1is a type II Golgi membrane glycoprotein that can be proteolytically processed to a soluble form. ST3Gal1 contributes to the alpha2-3-sialylation of O-glycans such as T antigens. Upon maturation of Dendritic cells derived from monocytes ST3GAl1 is altered in a stimulus-dependent manner. Multiple isoforms of human ST3GAL1 have been described. Human ST3GAL1 shares 85% identity with mouse ST3GAL1.