The authors describe an expression system for the correct, stable and effective expression in mammalian cells of a siRNA comprising: a) a polymerase II RNA-dependent promoter derived from the U1 RNA gene; downstream from the sequence b) suitable restriction sites for the cloning of the sequence transcribing the pre-siRNA; downstream from these sites c) sequences derived from the sequences at 3' of the gene for U1 snRNA that are necessary and sufficient for the correct formation of 3' of the presiRNA.