CD45 is the common leukocyte antigen and is expressed on all cells of hematopoietic origin except erythrocytes. CD45 exists as a number of different isoforms which result from alternative RNA splicing of exons 4, 5 and 6. The I3/2.3 monoclonal antibody recognizes a framework epitope present on all CD45 isoforms. The CD45 molecule has been implicated as playing a key role in thymic development and in antigen-driven proliferation of T cells.