CD45, also known as the leukocyte common antigen (LCA), is a transmembrane protein-tyrosine phosphatase (PTPase) that is expressed in almost all hematolymphoid cells including lymphocytes, granulocytes, monocytes and macrophages, but not in mature erythrocytes and megakaryocytes. CD45 antibody labelling of majority of hematolymphoid neoplasms, is a first line of marker for the identification of tumors with hematopoietic origin. Rare cases of undifferentiated and neuroendocrine carcinomas with CD45 positive staining have been reported.