CD34 is a type I transmembrane glycophosphoprotein expressed by hematopoietic stem/progenitor cells, vascular endothelium and some fibroblasts . CD34 expression has been the hallmark used to identify hematopoietic stem cells for many years. CD34+ hematopoietic stem cells expand and differentiate into all the lymphohematopoietic lineages upon cytokine or growth factor stimulation and lose CD34 expression upon differentiation. However, recent studies performed in various laboratories conflict with that convention. The extracellular domain of CD34 is homologous to CD43, a protein involved in cell-cell adhesion, and CD34 has been shown to function as a negative regulator of cell adhesion. CD34 associates with CrkL but not CrkII, is a substrate for PKC, and activation of PKC is coupled with surface expression of CD34.
