The polymorphic HUMTH01 microsatellite, located in the first intron of the tyrosine hydroxylase gene is characterized by a tetranucleotide core motif. The 10 repeat allele of this microsatellite exhibits two sequence variants: an imperfect repeat and a perfect repeat. Here we present evidence that this tetrarepeat is endowed with regulatory properties. Constructions were made linking the 10 repetition alleles to the luciferase reporter gene under the control of a thymidine kinase minimal promoter. In transient transfection experiments in HeLa, PC12 and SK-NSH cell lines these repeated sequences increased the basal transcription up to 9-fold. This effect was independent of the sequence orientation, a feature characteristic of an enhancer element. In electrophoretic mobility shift assays these tetrameric repeated sequences form specific complexes with HeLa cell nuclear extracts. Competition experiments with heterologous sequences suggest that proteins of the Fos-Jun family may be involved in the formation of these complexes, although other unidentified transacting factors bind to these sequences. These results thus implicate the HUMTH01 microsatellite in the regulation of tyrosine hydroxylase gene expression. Tetrarepeated sequences of this type may constitute a new class of regulatory elements.