Beta-1, 4-galactosyltransferase 1 (B4GALT1), one of seven beta-1, 4-galactosyltransferases, is an enzyme commonly found in the trans-Golgi complex that adds galactose to oligosaccharides. By sequence similarity, the B4GALTs form four groups: B4GALT1 and B4GALT2, B4GALT3 and B4GALT4, B4GALT5 and B4GALT6, and B4GALT7. B4GALT1 is unique among the seven enzymes because it can be expressed either as membrane associated form or secreted form. The secreted form is restricted to lactating mammary tissues where the enzyme forms a heterodimer with alpha – lactalbumin to catalyze the synthesis of lactose. The membrane form can reside either in the Golgi apparatus, where it adds galactose to N-acetylglucosamine residues, or on cell surface, where it functions as a recognition molecule during a variety of cell to cell and cell to matrix interactions, by binding to specific oligosaccharide ligands on opposing cells or in the extracellular matrix. The two enzymatic forms result from alternate transcription initiation sites and post-translational processing. Defects in B4GALT1 are the cause of congenital disorder of glycosylation type 2D.