CD34 is a type I monomeric sialomucin-like glycophophoprotein that is highly glycosylated and phosphorylated by protein kinase C. As CD34 protein is selectively expressed on hematopoietic progenitor cells it is extensively used as a molecular marker for identification, isolation and manipulation of hemopoietic stem cells and progenitors. The CD34 protein may play a role in the attachment of stem cells to the bone marrow extracellular matrix or to stromal cells. In addition to expression on hematopoietic progenitors, CD34 is expressed on some populations of mesenchymal stem cells, and by vascular endothelia in the adult. CD34 mediates cell adhesion and lymphocytes homing through binding to L-selectin and E-selectin ligands.