CD34 is a surface glycophosphoprotein expressed on developmentally early lymphohematopoietic stem and progenitor cells with a molecular weight of about 105-120 kD. It is selectively expressed on the majority of hematopoietic stem/progenitor cells,bone marrow stromal cells,capillary endothelial cells,embryonic fibroblasts,and some nerve tissue. CD34 is a commonly used marker for identifying human hematopoietic stem/progenitor cells and mediates cell adhesion and lymphocyte homing by binding L-selectin and E-selectin ligands. CD34 is also one of the best negative selection markers for characterizing and/or isolating human MSCs from bone marrow and other sources. Along with other positive selection markers (such as CD29,CD44,CD90,CD105 and CD166),negative selection markers (such as CD34 and CD45) are used for MSC identification.