CD45 is a type 1 transmembrane molecule found on the surface of all nucleated hematopoietic cells and their precursors, except for mature erythrocytes and platelets. It is one of the most abundant cell surface glycoproteins.The cytoplasmic region is 95% homologous across mammalian species. CD45 is expressed in various isoforms. Expression of different CD45 isoforms is cell type specific, and depends on the state of activation and differentiation of hematopoietic cells.