CD34 is a heavily glycosylated type I transmembrane molecule, that can be phoshorylated by a variety of kinases including Protein kinase C and Tyrosine kinases. The classification of epitopes detected by different CD34 MAbs has aided the selection of appropriate antibodies for use in specific clinical and research laboratory settings. Detailed structural analyses and cloning studies have confirmed that CD34 is a sialomucin, and have suggested that the fine composition of the carbohydrate moieties contained in its extended N-terminal region is important in determining its interactions with a variety of different ligands. For high endothelial venules (HEV) CD34 to serve as a ligand for L-selectin, the O-linked glycans of HEV CD34 are modified in an exquisitely specific manner with a variety of sialyl- and sulfo-transferases. In contrast, CD34 is not the ligand for L-selectin in hematopoietic stem/progenitor cells (HSPCs) and despite much endeavour, ligands for hematopoietic CD34 remain to be identified.