Human CC chemokine receptor 5 (CCR5), mediates the activation of cells by the chemokines macrophage inflammatory protein-1␣, macrophage inflammatory protein-1␤, and RANTES, and serves as a fusion cofactor for macrophage-tropic strains of human immunodeficiency virus type 1. To understand the molecular mechanisms that regulate human CCR5 gene expression, we initiated studies to determine its genomic and mRNA organization. Previous studies have identified a single CCR5 mRNA isoform whose open reading frame is intronless. We now report the following novel findings. 1) Complex alternative splicing and multiple transcription start sites give rise to several distinct CCR5 transcripts that differ in their 5-untranslated regions (UTR). 2) The gene is organized into four exons and two introns. 3) The transcripts appear to be initiated from two distinct promoters: an upstream promoter (P U ), upstream of exon 1, and a downstream promoter (P D ), that includes the "intronic" region between exons 1 and 3. 4) P U and P D lacked the canonical TATA or CAAT motifs, and are AT-rich. 5) P D demonstrated strong constitutive promoter activity, whereas P U was a weak promoter in all three leukocyte cell environments tested (THP-1, Jurkat, and K562). 6) We provide evidence for polymorphisms in the noncoding sequences, including the regulatory regions and 5-UTRs. The structure of CCR5 was strikingly reminiscent of the overall structure of other chemokine/chemoattractant receptors, underscoring an important evolutionarily conserved function for a prototypical gene structure. This is the first description of functional promoters for any CC chemokine receptor gene, and we speculate that the complex pattern of splicing events and dual promoter usage may function as a versatile mechanism to create diversity and flexibility in the regulation of CCR5 expression.CC chemokine receptor 5 (CCR5), 1 a receptor for the CC chemokines macrophage inflammatory protein-1␣, macrophage inflammatory protein-1␤, and RANTES (1-3), also serves as a fusion cofactor for the entry of macrophage-tropic strains of HIV-1 (4 -8). The level of CCR5 cell surface expression may have a direct influence on the relative ease with which an individual acquires HIV-1 infection (9 -12): individuals homozygous for a 32-bp deletion (denoted ⌬CCR5) in the open reading frame (ORF) do not express the protein on the cell surface, and are relatively resistant to developing HIV-1 infection. In contrast, individuals who display the CCR5/⌬CCR5 genotype can develop HIV-1 infection, however, their progression to AIDS may be slower. Interestingly, in individuals who display the CCR5/CCR5 genotype, the cell surface expression of CCR5 can be highly variable (13), however, whether this heterogeneity in protein expression also correlates with differences in HIV-1 infection/transmission in vivo is not known. These observations suggest that a therapeutic or preventive strategy based on targeting CCR5 cell surface expression could potentially be quite beneficial. Toward this end, we have initia...