Human CC chemokine receptor 5 (CCR5), which is 17.5 kb from the CMKBR2 gene, encodes a 352-amino acid protein with a calculated molecular mass of 40,600 Da. It is organized into four exons and two introns. Human CCR5 is a co-receptor for macrophage-tropic strains of human immunodeficiency virus (HIV)-1 and is expressed by bone-marrow-derived cells. CCR5 is expressed on neurons, astrocytes, and microglia in the central nervous system, and on epithelium, endothelium, vascular smooth muscle and fibroblasts in other tissues. Functionally, CCR5 mediates the activation of cells by the chemokines macrophage inflammatory protein-1alpha, macrophage inflammatory protein-1beta, and RANTES, and serves as a fusion cofactor for macrophage-tropic strains of human immunodeficiency virus type 1.