CD45 (Leukocyte Common Antigen) is a protein family composed of five or more high molecular weight glycoproteins, mainly expressed on the surface of leukocytes, including T, B lymphocytes, polymorphonuclear leukocytes, monocytes, etc., but not expressed in erythrocytes, platelets and non hematopoietic systems. It is a specific marker to distinguish lymphoma leukaemia from non hematopoietic tissue tumors (such as undifferentiated small cell carcinoma and small round cell sarcoma).