Short interfering RNAs (siRNAs) are double-stranded RNAs of approximately 19-29 nucleotides designed to suppress the expression of homologous genes by a process known as RNA interference (RNAi). In this study we have characterized several short interfering RNAs to reduce (knockdown) the expression of genes related to the dopaminergic system of the ventral mesencephalon. We report here effective suppression of all targeted genes, tyrosine hydroxylase (TH), the transcription factor Nr4a2 (Nurr1) and the GDNF receptor moiety cRet, by co-transfection of plasmids expressing gene specific siRNAs under control of the human U6 promoter with a reporter plasmid coding for firefly luciferase and fused in tandem to the cDNA for the gene of interest. The effective suppression of Nr4a2, cRet and tyrosine hydroxylase suggest that the U6 expression cassette could be used to deliver siRNA to mammalian cells in vivo and actively suppress the expression of dopamine related mammalian genes. The characterization of highly effective siRNAs for DA phenotypic markers may open new avenues for loss-of-function phenotypic analysis, and may lead to new approaches for gene therapy.