I-309 is a CC chemokine that is produced by activated T lymphocytes, monocytes, and a mast cell leukemia. It has structural features that distinguish it from other chemokines, including a third intrachain disulfide bond and a monomeric association state even at high
concentrations. I-309 binds selectively to CCR8 and is chemotactic for monocytes and TH2-differentiated T cells in vitro.