CCL17, also known as thymus and activation-related chemokine (TARC), a small cytokine belonging to the C-C chemokine family. Among CC chemokine family members, CCL17 has approximately 24 – 29% amino acid sequence identity with RANTES, MIP-1 alpha, MIP-1 beta, MCP-1, MCP-2, MCP-3 and I-309. CCL17 is constitutively expressed in thymus, and at a lower level in lung, colon, and small intestine. CCL17 is also transiently expressed in phytohemagglutinin-stimulated peripheral blood mononuclear cells. CCL17 has been shown to be chemotactic for T cell lines but not monocytes or neutrophils. CCL17 was identified to be a specific functional ligand for CCR4, a receptor that is selectively expressed on T cells.