The 581 antibody reacts with human CD34, 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 mesenchymal stem cells, embryonic fibroblasts, endothelial cells, neurons, and some tumor cell lines. CD34 is expressed only transiently during hematopoiesis, so the frequency of CD34+ cells is low in bone marrow or cord blood (~1 - 5%) and very low (~0.1 - 0.5%) in peripheral blood. CD34 is a marker used to identify and isolate HSPCs capable of cell engraftment. CD34 is thought to mediate attachment of stem cells to the bone marrow extracellular matrix or directly to stromal cells during early hematopoiesis, and to be involved in lymphocyte recruitment through binding to the ligands L- and E-selectin. Distinct epitope groups have been assigned to CD34 based on their sensitivity to enzymatic cleavage, with the 581 antibody recognizing a class III epitope (resistant to neuraminidase and O-glycoprotease). This antibody clone has been verified for purity assessments of cells isolated with EasySep™ Human CD34 Positive Selection Kit (Catalog #18056) and for labeling human mesenchymal cells grown in MesenCult™ Proliferation Kit (Human; Catalog #05411).