CD45 is a transmembrane glycoprotein expressed on most nucleated cells of hematopoetic origin. On human leucocytes, five different isoforms of CD45 have been identified, named ABC, AB, BC, B and 0. This antibody reacts with an epitope unique for CD45R0. The antibody labels most thymocytes, a subpopulation of resting T cells within both CD4 and CD8 subsets, and mature, activated T cells. It is effective on formalin-fixed, paraffin-embedded tissue sections. Results aid in the classification of T-cell neoplasms.