CD45 (LCA, PTPRC) is a type 1 transmembrane protein tyrosine phosphatase that is a marker for leukocytes. It is expressed on almost all hematopoietic cells (B and T-lymphocytes, granulocytes, monocytes, mast cells, dendritic cells, and macrophages). It is positive in many hematolymphoid neoplasms and may assist in the classification of lymphomas and leukemias. It is negative in mature erythrocytes, platelets, plasma cells, and mature megakaryocytes. Defects in PTPRC are one cause of severe combined immunodeficiency syndrome characterized by impairment of both humoral and cell-mediated immunity, leukopenia, and low or absent antibody levels. References: Hermiston et al. Annual review of immunology 21.1 (2003): 107-137, DOI: 10.1146/annurev.immunol.21.120601.140946;Am J Clin Pathol 1998;110:797, PMID: 9844593