Chemokine ligand (CCL17), also known as thymus activation-regulated chemokine (TARC), is a small molecular weight cytokine of the CC chemokine family.The gene encoding CCL17 is located on chromosome 16 in humans, along with the other chemokines CCL22 and CX3CL1.CCL17 is constitutively expressed in the thymus, and transiently expressed in phytohaemagglutinin (PHA)-stimulated peripheral blood individual nuclei. nuclei are transiently expressed in peripheral blood mononuclear cells stimulated by phytohemagglutinin (PHA). This chemokine specifically binds to T cells and exhibits chemotaxis by binding to the chemokine receptor CCR4.