B4GALT1 (beta-1,4-galactosyltransferase 1) in the Golgi membrane associates with LALBA (alpha-lactalbumin) to form a heterodimer. This binding reaction is positively regulated by Mn2+ ions and UDP-galactose. This heterodimer has been observed both in solution in vitro and in crystalline form (Andrews 1970; Ramakrishnan & Qasba 2001). The galactosyltransferase enzyme is found in the Golgi membrane of many cell types. LALBA is expressed specifically in the lactating mammary gland. Binding LALBA modulates the specificity of the galactosyltransferase enzyme. Alone, B4GALT1 transfers galactose moieties to glycolipids and N-linked oligosaccharides of glycoproteins. Complexed with LALBA, it transfers galactose to free glucose to form lactose (Brew and Hill 1975).