Figure 4. Model of CD4 dimer conformations and their hypothetical difference in signal transduction via p56lck. Usually, monomeric form of CD4 receptors is prevailing, but they can also dimerize, to some extent, through interactions of their D1 or D4 domains, resulting in two distinct dimer conformations. As crystallographic studies had only revealed the conformation B, it could be the predominant form of dimers. Critical residues in the dimer interfaces are depicted. This model suggest a different relative position of the cytoplasmic tails in the two CD4 dimer conformations that could affect their capacity to activate p56lck and induce nuclear translocation of AP-1 and NF-κB.