This group belongs to the rhodopsin class of G protein-coupled receptors. They are found on many different cell types, including immune, endothelial, epithelial, and neuronal cells. The ligands for these receptors are chemokines, which are small, 8-10 kDa proteins best known for their ability to direct cell migration in a process known as chemotaxis. Chemotaxis is very important during embryogenesis and organogenesis, and during homing of leukocytes to, and within, lymphoid organs. Furthermore, chemotaxis via chemokines is crucial for leukocyte trafficking to sites of inflammation. Chemokine receptor signaling is involved other diverse cellular processes as well, including degranulation of immune cells, cell proliferation, angiogenesis, and apoptosis. In addition, chemokine receptors CXCR4 and CCR5 serve as co-receptors with CD4 for entry of the HIV virus. The receptor/ligand specificity of this system is variable, as some chemokine receptors can be activated by only one chemokine, while other receptors may be activated by many different chemokines.