Among the various posttranslational modification reactions, glycosylation is the most common, and nearly 50% of all known proteins are thought to be glycosylated. In particular, most of the molecules involved in cell-cell communication are glycosylated, and glycosylation is thus implicated in many physiological and pathological events, including cell growth, cell-cell adhesion, and tumor metastasis. As many of the glycosyltransferases are cloned, it is becoming possible to alter the oligosaccharide structures artificially and examine the effects. Among the glycosyltransferases involved in the biosynthesis of N-glycan branching, this review will focus on the function of Fut8 and N-acetylglucosaminyltransferase III, which directly modify the N-glycan core. It is suggested that these two glycosyltransferases are involved in the conformation and the function of the modified proteins including cell-surface receptors and adhesion molecules.