Cluster of differentiation 5 (CD5) is expressed in high levels on the surface of T cells, while controversy surrounds the expression levels and role of CD5 in B cells. As a part of a diagnostic panel, its utility lies predominantly as a marker for T cells, with over 70% of T cell neoplasms expressing CD5. In particular, it is correlated with chronic lymphocytic leukemia/small lymphocytic lymphomas, mantle cell lymphoma, as well as a subset of diffuse large B cell lymphomas. CD5 demonstrates positive expression in thymic carcinomas, and is not as sensitive as CD3. CD5 also has value as a prognostic indicator, being associated with poor prognosis in acute T cell lymphoblastic leukemia.

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