CD4
CD4 (cluster of differentiation 4) is a molecule that is expressed on the surface of T helper cells (as well as regulatory T cells and dendritic cells). On T cells, CD4 is the co-receptor for the T cell receptor (TCR) and recruits the tyrosine kinase lck. CD4 is also a coreceptor, along with the chemokine receptors CCR5 or CXCR4, for the HIV virus.