β-1,4-galactosyltransferase 1 (B4GALT1) is one of seven β-1,4-galactosyltransferases that are typically found in the trans-Golgi complex and transfer galactose to oligosaccharides. Based on sequence similarity, B4GALTs can be divided into four groups: B4GALT1 and B4GALT2, B4GALT3 and B4GALT4, B4GALT5 and B4GALT6, and B4GALT7. Among these enzymes, B4GALT1 is unique in that it can be expressed either as a membrane-bound or secreted form. The secreted form is only found in lactating mammary tissue where it forms a heterodimer with α-lactalbumin to catalyze lactose synthesis. The membrane-bound form is either found in the Golgi where it adds galactose to N-acetylglucosamine residues, or on the cell surface where it acts as a recognition molecule by binding to specific oligosaccharide ligands on opposing cells or the extracellular matrix during cell-cell or cell-matrix interactions. These two forms of the enzyme are produced from alternate transcription start sites and post-translational processing.Deficiencies of B4GALT1 can lead to congenital disorders of glycosylation (CDG) type 2D, which is characterized by defective N-glycosylation.