CD4 is a 55 kD glycoprotein expressed on the surface of T-helper/regulatory T-cells, monocytes, macrophages, and dendritic cells. Anti-CD4 is used in the immunophenotyping of lymphoproliferative disorders. The majority of peripheral T-cell lymphomas are derived from the T-helper/regulatory cell subset so that most mature T-cell neoplasms are CD4+ CD8-. As with other T-cell antigens, CD4 may be aberrantly expressed in neoplastic T-cells so that the evaluation of such tumors requires the application of a panel of markers in order to identify tumors with CD4 aberrant expression.