The CD45 molecules are single chain integral membrane proteins, comprising at least 5 isoforms, ranging from 180 to 220 kDa. They are generated by alternative splicing combinations of three exons (A, B, and C) of the genomic sequence. The non-restricted CD45 antigen, Leucocyte Common Antigen (LCA) consists of an extracellular sequence, proximal to the membrane, which is common to all CD45 isoforms. All the monoclonal antibodies that belong to the CD45 cluster react with this part of the antigen and are able to recognize all CD45 isoforms. These isoforms have extracytoplasmic sequences ranging from 391 to 552 amino acids long, with numerous N-linked carbohydrate attachment sites. The cytoplasmic portion contains two phospho-tyrosine-phosphatase domains. The non-restricted CD45 epitope is present on the surface of all human leucocytes; lymphocytes, eosinophils monocytes, basophils and neutrophils, by order of decreasing level of expression. CD45 is a major component of the lymphocyte membrane. It is absent from erythrocytes and platelets. It is lost during maturation of erythroid cells in the bone marrow CD45 antibodies react with leucocyte progenitors in bone marrow.