There are four members of the chemokine family: C-C kemokines, C kemokines, CXC kemokines and CX3C kemokines. The C-C kemokines have two cysteines nearby the amino terminus. There have been at least 27 distinct members of this subgroup reported for mammals, called C-C chemokine ligands-1 to 28. Chemokin ligand 17 (CCL17), also known as thymus and activation regulated chemokine(TARC), is a small cytokine belonging to the C-C chemokine family. CCL17 is expressed maily in thymus and transiently in phytohemagglutinin-stimulated peripheral blood mononuclear cells. CCL17 can induce chemotaxis in T cells by binding with the chemokine receptor CCR4.