CCL17 is a novel CC chemokine identified using a signal sequence trap method. CCL17 cDNA encodes a highly basic 94 amino acid (aa)residue precursor protein with a 23 aa residue signal peptide that is cleaved to generate the 71 aa residue mature secreted protein. 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. The gene for human CCL17 has been mapped to chromosome 16q13 rather than chromosome 17 where the genes for many human CC chemokines are clustered. CCL17 is constitutively expressed in thymus, and at a lower level in lung, colon, and small intestine. CCL17 is also transiently expressed in stimulated peripheral blood mononuclear cells. Recombinant 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.