CCR5 gene organization and genotypes studied. The CCR5 gene is on chromosome 3p21 and is a 3-exon gene (filled boxes in the CCR5 gene diagram) with a proximal and distal promoter region (not shown). The relative positions of the CCR5 variants used to determine the CCR5 H1/H1 genotype are shown. The entire open reading frame of the gene is on exon 3 (filled box in the CCR5 mRNA diagram) and the majority of transcripts originate in exon 2 from the proximal promoter. Thus, the first 2 SNPs are promoter polymorphisms in the proximal promoter, the third SNP is in the intron, and fourth is the 32-basepair deletion in the open reading frame. Different nomenclatures have been used for the same position and genotype in different publications,^19-22  so a table is provided with these designations and the nucleotide sequence that defines the CCR5 H1/H1 genotype in this study.