CD4 (cluster of differentiation 4) is a glycoprotein expressed on the surface of T helper cells, regulatory T cells, monocytes, macrophages, and dendritic cells. It is a member of the immunoglobulin superfamily. On T cells, CD4 is the co-receptor for the T cell receptor (TCR) and amplifies the signal generated by the TCR by recruiting the tyrosine kinase lck that is essential for activating many molecules involved in the signaling cascade of an activated T cell. CD4 is an important component of the immune system and is also the cellular receptor for HIV-1. It consists of a cytoplasmic tail, one transmembrane region, and four extracellular domains, D1-D4. It maps to 12pter-p12 region of human chromosome.