O-GlcNAcylation of proteins plays a critical role in cell cycle regulation, apoptosis and signal transduction. Modification of O-GlcNAcylation of proteins by O-GlcNAc transferase is reversible by the activity of O-GlcNAcase. Addition and removal of O-GlcNAc moiety to serine or threonine regulate the function of proteins directly or via decreased phosphorylation. O-GlcNAcylation of c-myc proteins induces ubiquitin-dependent c-myc degradation. GlcNAcylation of p53 prevents p53 degradation and stabilizes p53 proteins, which are beneficial in cancer cells by arresting cell growth and inducing apoptosis in response to DNA damage. To elucidate functions of O-GlcNAcylation of the proteins it is necessary to develop site-specific antibodies for O-GlcNAcylated proteins. Few site-specific antibodies have been successfully produced so far. A lack of site-specific O-GlcNAcylated Abs prevents facile detection and quantitation of the site to elucidate the function of O-GlcNAcylated molecules in physiology and disease. Availability of O-GlcNAcylation site-specific antibodies will facilitate mechanistic studies on O-GlcNAcylation ? and phosphorylation-dependent signal transduction pathway in cancer, which will result in development of O-GlcNAcylation-inducing anti-cancer drugs.