Chemokine (C-C motif) ligand 17 (CCL17) is a small cytokine belonging to the CC chemokine family is also known as thymus and activation regulated chemokine (TARC). CCL17 is expressed constitutively in thymus, but only transiently in phytohemagglutinin-stimulated peripheral blood mononuclear cells. This chemokine specifically binds and induces chemotaxis in T cells and elicits its effects by interacting with the chemokine receptor CCR4. The gene for CCL17 is located on chromosome 16, in humans, along with other chemokines called CCL22 and CX3CL1.

TARC is an extremely useful clinical biomarker for atopic dermatitis (AD) treatment. Usually, physicians conduct a visual examination to determine whether their treatment has been successful; however, the visual examination results may not always be accurate; in such cases, serum TARC levels should be measured to eliminate any ambiguity regarding the treatment outcome.