CD45, also known as leukocyte common antigen, is a specific marker of hematopoietic cells. CD45 consists of at least four isoforms produced by alternative messenger RNA splicing. The isoforms share a common large intracellular domain and the extracellular domains are rod shaped, heavily glycosylated. It is expressed in all T cells, B cells and NK cells except plasma cells, monocytes, granulocytes and macrophages, but not in erythrocytes, platelets and non hematopoietic systems, Therefore, it can be used as a good marker to distinguish lymphoma or leukemia from non hematopoietic tumors.