Genetic studies have implicated a polymorphic repeat sequence in exon 3 of the human dopamine D4 receptor in various behavioral and psychiatric disorders. Functionally various repeat variants are nearly identical, but whether these have different effects on gene expression has not been studied. To study the role of the repeat sequences on expression independently from its structural and functional effects at the protein level, we introduced these sequences immediately upstream of the promoter and in the 3' untranslated region of a luciferase reporter vector. In this report, we demonstrate that the repeat sequence can both modulate promoter activity and alter expression post-transcriptionally. The repeat sequence can serve as a substrate for a nuclear binding factor and all the three repeat variants can suppress promoter activity. Placement of the three repeat variants downstream from the luciferase gene in the expression vector shows, however, that the D4.7 repeat sequence has significantly suppressed expression of the reporter compared to the D4.2 and D4.4 repeats, likely via mechanisms involving RNA stability or translational efficiency. These data indicate that the various D4 repeat sequences have different effects on expression, which may explain its potential role in behavioral disorders.