CD34 is an ~105 – 120 kDa type 1 transmembrane glycoprotein expressed on the surface of most human hematopoietic stem and progenitor cells (HSPCs), as well as on endothelial cells and some tumor cell lines. The frequency of CD34+ cells is low in bone marrow (1 – 4%), cord blood (~1%), and peripheral blood (< 0.1%). CD34 is used to identify and isolate HSPCs, including those capable of reconstituting hematopoiesis after transplantation. Distinct epitope groups have been assigned to CD34 based on differences in the structure of carbohydrate groups, which affects their sensitivity to enzymatic cleavage. The 563 antibody recognizes a class III epitope (resistant to neuraminidase and O-glycoprotease). Clone 563 also cross-reacts with CD34+ cells of rhesus, cynomolgus, and pigtailed macaques, but not baboons.