The Golgi complex form catalyzes the production of lactose in the lactating mammary gland and could also be responsible for the synthesis of complex-type N-linked oligosaccharides in many glycoproteins as well as the carbohydrate moieties of glycolipids. The cell surface form functions as a recognition molecule during a variety of cell to cell and cell to matrix interactions, as those occurring during development and egg fertilization, by binding to specific oligosaccharide ligands on opposing cells or in the extracellular matrix. D-glucose + UDP-alpha-D-galactose = H(+) + lactose + UDP an N-acetyl-beta-D-glucosaminyl derivative + UDP-alpha-D-galactose = a beta-D-galactosyl-(1->4)-N-acetyl-beta-D-glucosaminyl derivative + H(+) + UDP N-acetyl-D-glucosamine + UDP-alpha-D-galactose = beta-D-galactosyl-(1->4)-N-acetyl-D-glucosamine + H(+) + UDP a beta-D-GlcNAc-(1->3)-beta-D-Gal-(1->4)-beta-D-Glc-(1<->1)-Cer(d18:1(4E)) + UDP-alpha-D-galactose = a neolactoside nLc4Cer(d18:1(4E)) + H(+) + UDP beta-D-glucosylceramide + UDP-alpha-D-galactose = beta-D-galactosyl-(1->4)-beta-D-glucosyl-(1<->1)-ceramide + H(+) + UDP a neolactoside IV(3)-beta-GlcNAc-nLc4Cer + UDP-alpha-D-galactose = beta-D-galactosyl-(1->4)-N-acetyl-beta-D-glucosaminyl-(1->3)-beta-D-galactosyl-(1->4)-N-acetyl-beta-D-glucosaminyl-(1->3)-beta-D-galactosyl-(1->4)-beta-D-glucosyl-(1<->1')-ceramide + H(+) + UDP Protein modification; protein glycosylation. Homodimer; and heterodimer with alpha-lactalbumin to form lactose synthase. Interacts (via N-terminal cytoplasmic domain) with UBE2Q1 (via N-terminus); the interaction is direct (By similarity). Found in trans cisternae of Golgi but is mainly localized at the plasma membrane (PubMed:1714903). B4GALT1 cell surface expression is regulated by UBE2Q1 (By similarity). Found in trans cisternae of Golgi. Soluble form found in body fluids. Ubiquitously expressed, but at very low levels in fetal and adult brain. The soluble form derives from the membrane forms by proteolytic processing. The disease is caused by variants affecting the gene represented in this entry. Belongs to the glycosyltransferase 7 family. Beta-1,4-galactosyltransferase 1