Thymus and activation-regulated chemokine (TARC) is a CC chemokine constitutively expressed most prominently in the thymus and also to a lesser extent in tissues such as lung, colon, and small intestine. TARC was the first CC chemokine shown to be chemotactic for lymphocytes but not for monocytes. It is produced by dendritic cells in various tissues and also by dendritic cells differentiated from blood monocytes in vitro. TARC and MDC, another CC chemokine that is very similar to TARC, act on CCR4. In the thymus, CCR4 is expressed mainly on CD4+ thymocytes in the cortex. Thus, TARC and MDC, which are expressed in the medullary dendritic cells and epithelial cells respectively, may be involved in guiding the migration of positively selected CD4+ thymocytes from cortex to medulla for negative selection. In peripheral blood, CCR4 is expressed by a subpopulation of CD4+CD45RO+ memory T cells, which are mostly if not exclusively polarized to TH2. Thus, it is likely that TARC and MDC play important pathophysiological roles in TH2-type immune responses.