CD4, also known as T4/Leu-3, is a 55 kD single-chain type I transmembrane glycoprotein and member of the immunoglobulin superfamily. It is expressed on most thymocytes, helper T cells, type II NKT cells, and monocytes/macrophages. CD4 is part of the TCR/CD3 complex, binds to β2 domain from the MHC class II molecule, and participates in TCR signal transduction. CD4 is the receptor of IL-16 and is a coreceptor for the human immunodeficiency virus (HIV) and human herpes virus 7 (HHV-7).