CD34 is a heavily glycosylated, 105-120 kDa transmembrane glycoprotein expressed on hematopoietic progenitor cells, vascular endothelial cells and some fibroblasts. The CD34 cytoplasmic domain is a target for phosphorylation by activated protein kinase C, suggesting a role for CD34 in signal transduction. CD34 may also play a role in adhesion of certain antigens to endothelium.