Cluster of differentiation 4 (CD4) is a membrane glycoprotein expressed in T helper cells, monocytes, macrophages, granulocytes, and dendritic cells and is a receptor of human immunodeficiency virus (HIV). CD4 staining is used for identifying lymphoproliferative disorders. Since the majority of peripheral T-cell lymphomas arise from the T helper cell subset, CD4 expression can be found on most forms of T-cell lymphomas as well as anaplastic large T cell lymphomas and mycosis fungoides. Since CD4 may be aberrantly expressed in neoplastic T-lymphocytes, a panel of markers may be used to identify such tumours. CD4(+) CD25(+) T cells are reported to exert immunosuppression, which is commonly observed in various types of cancers, including non-small cell lung cancer, and cancers of the breast, prostate, and ovary.