Monocyte chemoattractant protein-4 (MCP-4) is a CC chemokine implicated in the recruitment of eosinophils, monocytes, and T-lymphocytes in diseases of mucosal inflammation, including asthma. We tested the hypothesis that there is a genetic basis for differences in monocyte chemoattractant protein-4 (MCP-4) expression among individuals by evaluating the effects of core promoter variants on MCP-4 expression. We identified two single-nucleotide T-to-C polymorphisms in the MCP-4 core promoter that occur 896 and 887 base pairs preceding the transcription initiation site. The -887 variant alters a consensus binding motif for the transcription factor YY-1. Electrophoretic mobility shift assay (EMSA) demonstrated that YY-1 containing nuclear extracts from tumor necrosis factor a-stimulated peripheral blood mononuclear cells had greater avidity for the wild-type (YY-1 motif intact) sequence than for the variant sequence. Increasing doses of a YY-1 expression vector induced significantly greater reporter activity from MCP-4 core promoter expression constructs of the wild type compared with the variant sequence in transient transfection experiments. The external validity of these observations was demonstrated by measuring plasma levels of MCP-4 from individuals with the alternative forms of the gene. Individuals bearing haplotypic variants of the MCP-4 core promoter that avidly bind the transcription factor YY-1 had higher plasma levels of MCP-4 than did individuals with variants with lower binding avidity (490; 360; and 360 pg/ml; P<0.01). Our findings suggest that the MCP-4 core promoter YY-1 binding motif is functional, modulates the transcriptional regulation of the MCP-4 gene, and that part of the variance in the systemic expression of MCP-4 is determined by core promoter genetic variants.