Cluster of Differentiation 34 (CD34) is a member of a family of single-pass transmembrane sialomucin proteins, and may function as a cell-cell adhesion factor. CD34 protein is selectively expressed on hematopoietic progenitor cells and the small vessel endothelium of a variety of tissues. It has been widely used as a stem and progenitor cell marker, and clinical CD34+ stem cell transplantation (CD34+SCT) has been performed for tumor purging. CD34 monoclonal antibodies are widely used to identify and isolate hemopoietic progenitors and to classify acute and chronic leukemias.