CD4 is an approximately 55?kDa type?I transmembrane glycoprotein that is expressed predominantly on thymocytes and a subset of mature T?lymphocytes. It is a standard phenotype marker for the identification of T?cell populations. Mature human CD4 consists of a 371?amino acid extracellular region containing four immunoglobulin-like domains, a 22?aa transmembrane segment, and a 40?aa cytoplasmic domain. CD4 is expressed along with CD8 on double positive T?cells during their development in the thymus. CD4 binds directly to MHC class?II molecules on antigen presenting cells (10). This interaction contributes to the formation of the immunological synapse which is focused around the TCR-MHC class II-antigenic peptide interaction. CD4 also functions as a chemotactic receptor for IL-16 and, in human, as a co-receptor for the gp120 surface glycoprotein of HIV-1.