CD45 is a family of single-chain transmembrane glycoproteins, consisting of at least four isoforms that 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 most haematolymphoid neoplasms, i.e., leukemias and malignant lymphomas. Overall, about 90% of malignant lymphomas are CD45 positive. CD45 is an essential marker in the primary tumor screening panel to identify haematolymphoid differentiation. Loss of CD45 in precursor B-cell neoplasms is a negative prognostic parameter.