CD34 is a 115-kDa transmembrane glycoprotein expressed on the surface of hematopoietic progenitor cells and small blood vessel endothelial cells. While the specific function of CD34 remains to be elucidated, it is regarded as a marker of hematopoietic stem cells (HSC) and hematopoietic progenitor cells. Cells positive in CD34, or CD34+, represents a mixed sample of mostly hematopoietic progenitors with some HSC. CD34+ cells are mainly found in the blood, umbilical cord and bone marrow, but are not limited to these tissues. CD34 is also expressed on nonhematopoietic cell types, including muscle satellite cells, keratocytes, interstitial cells, fibrocytes, epithelial progenitors, and endothelial cells.