Test Details: CD34 is a transmembrane phosphoglycoprotein protein commonly used as a marker of hematopoietic progenitor cells and endothelial cells. It may mediate attachment of hematopoietic stem cells to bone marrow extracellular matrix or directly to stromal cells. Flow cytometric enumeration of CD34+ cells is commonly employed to assess the hematopoietic stem cell (HSC) numbers in peripheral blood, cord blood, and apheresis products used for peripheral blood stem cell (PBSC) transplantation