Aberrant glycosylation of proteins and lipids is one of the characteristic features of malignantly transformed cells. The GalNAc β 1 → 4GlcNAc (LacdiNAc or LDN) group at the nonreducing termini of both N- and O-glycans is not generally found in mammalian cells. We previously showed that the expression level of the LacdiNAc group in N-glycans decreases dramatically during the progression of human breast cancer. In contrast, the enhanced expression of the LacdiNAc group has been shown to be associated with the progression of human prostate, ovarian, and pancreatic cancers. Therefore, the expression of the disaccharide group appears to be dependent on types of tumors. The mechanism of formation of the LacdiNAc group in human tumors and cancer cells has been studied, and two β 4-N-acetylgalacto-saminyltransferases ( β 4GalNAcTs), β 4GalNAcT3 and β 4GalNAcT4, have been shown to be involved in the biosynthesis of this disaccharide group in a tissue-dependent manner. Transfection of the β 4GalNAcT3 gene brought about significant changes in the malignant phenotypes of human neuroblastoma, indicating that this disaccharide group is important for suppressing the tumor growth.