CD45 is a family of single chain transmembrane glycoproteins, consisting of at least four isoforms which share a common large intracellular domain. Their extracellular domains are rod shaped, heavily glycosylated. The different isoforms are produced by alternative messenger RNA splicing of three exons of a single gene on chromosome 1q31-32.CD45 is exclusively expressed in haematolymphoid cells. Almost all haematolymphoid cells, including precursor cells and mature B- and T-lymphocytes, granulocytes, monocytes/histiocytes and interdigitating reticulum cells and follicular dendritic cells express CD45. This protein is detected in the large majority of haematolymphoid neoplasms, i.e., leukemias and malignant lymphomas. Overall, about 90% of malignant lymphomas are CD45 positive.CD45 is an important marker in the primary tumor screening panel in order to identify haematolymphoid differentiation. Loss of CD45 in precursor B-cell neoplasms is a negative prognostic parameter.