CCL1 or chemokine (C-C motif) ligand 1, also known as I-309 or TCA-3, is a member of the chemokine (C-C motif) ligand family. The C-C chemokines have two cysteines nearby the amino terminus. There have been at least 27 distinct members of this subgroup reported for mammals, called C-C chemokine ligands (CCL)-1 to 28. I-309/CCL1/TCA-3 interacts with the G protein-linked transmembrane chemokine receptors CCR8 and induces biochemical events that may result in the control of chemotaxis, proliferation, apoptosis and adhesion. It has been demonstrated that I-309/CCL1/TCA-3 displays chemotactic activity for monocytes and other cell types such as NK cells and dendritic cells, but not for neutrophils. Furthermore, as the only known physiological ligand for CCR8, I-309/CCL1/TCA-3 was identified as a potent inhibitor of HIV-1 envelope-mediated cell-cell fusion and virus infection. I-309/CCL1/TCA-3 induces significant levels of LTC4 from elicited eosinophils.