CCL5, also known as RANTES (Regulated upon Activation Normal T cell Expressed and Secreted), is an 8kDa β-chemokine that plays a primary role in the inflammatory immune response by means of its ability to attract and activate leukocytes. Human and mouse RANTES exhibit cross-species activity on human and mouse cells. CCL5 is secreted by many cell types at inflammatory sites and it exerts a wide range of activities through the receptors CCR1, CCR3, CCR4 and CCR5. In humans, CCR5 binding to CCL5 inhibits the infectivity of R5 (M-tropic) but not X4 (T-tropic) strains of HIV-1. The deposition of CCL5 on activated vascular endothelial cells is crucial for monocyte adhesion to damaged vasculature, but CCL5 oligomerization is not required for the extravasation of adherent leukocytes. CCL5 is upregulated in breast cancer and promotes tumor progression through the attraction of proinflammatory macrophages in addition to its actions on tumor cells, stromal cells and the vasculature.