Chemokines are a family of small chemotactic cytokines, or proteins secreted by cells. Chemokines share the same structure similarities such as small size, and the presence of four cysteine residues in conserved locations in order to form their 3-dimensional shape. Some of the chemokines are considered pro-inflammatory which can be induced to recruit cells of the immune system to a site of infection during an immune response, while others are considered homeostatic and are implied in controlling the migration of cells during normal processes of tissue maintenance and development. 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.