Cluster of differentiation 45 (CD45), also known as leukocyte common antigen (LCA), is a member of the protein tyrosine phosphatase (PTPase) family which is known to regulate a variety of cellular processes including cell growth, differentiation, the mitotic cycle, and oncogenic transformation. CD45 is expressed is expressed in almost all hematolymphoid cells including lymphocytes, granulocytes, monocytes and macrophages, but not in mature erythrocytes and megakaryocytes, and is an essential regulator of T and B cell antigen receptor signalling. Anti-CD45 positively stains the majority of lymphoid neoplasms, and is highly indicative of lymphoid origin. However, an absence of CD45 does not rule out lymphoid tumors, as certain types of neoplasms lack CD45, such as Hodgkin's lymphoma, some T-cell lymphomas, and some leukemias. Rare cases of undifferentiated and neuroendocrine carcinomas with CD45 positive staining have been reported.