The galE gene product, UDP-galactose 4-epimerase, mediates the incorporation of galactose in extracellular polysaccharide materials such as the O-side chain of lipopolysaccharide (LPS). The O-side chain in H. pylori LPS has been shown to cross-react with Lewis x and/or y blood group antigens, suggesting its potential involvement: in H. pylori-linked autoimmune disease. To study its role in H. pylori LPS biosynthesis, the galE gene was cloned. sequenced, and a galE-knockout H. pylori strain was constructed. The H. pylori galE gene encoded a protein of 344 amino acids with a molecular weight of 39K. The LPS profile from the galE-knockout H. pylori strain showed a lower molecular weight than that of the parental strain, indicating the involvement of the galE gene in LPS biosynthesis of H. pylori.