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CD34 is a transmembrane phosphoglycoprotein first identified on hematopoietic stem and progenitor cells. It is a member of the sialomucin family, with other members including podocalyxin and endoglycan. This protein functions primarily as an adhesion molecule, involved in cell-cell adhesion and cell-matrix interactions. It enhances the proliferation of progenitor cells and blocks cell differentiation, which is crucial for maintaining a pool of immature cells that can develop into various specialized cells as needed. CD34 also plays a role in immune system regulation, particularly in the adhesion of lymphocytes to blood vessel walls. Besides hematopoietic cells, CD34 is expressed on endothelial cells, fibrocytes, muscle satellite cells, and some cancer stem cells. Its expression has been utilized for the selection and enrichment of hematopoietic stem cells for bone marrow transplants.