Methods and means are provided for reducing the phenotypic expression of a nucleic acid of interest in eucaryotic cells, particularly in plant cells, by introducing chimeric genes encoding sense and antisense RNA molecules directed towards the target nucleic acid, which are capable of forming a double stranded RNA region by base-pairing between the regions with sense and antisense nucleotide sequence or by introducing the RNA molecules themselves. Preferably, the RNA molecules comprises simultaneously both sense and antisense nucleotide sequence.