InvivoGen

InvivoGen has designed an efficient and simple-to-use vector, called psiRNA, that allows the production of siRNAs within the cells. psiRNA is used to insert a DNA fragment of approximately 50 mer designed in such a way that after transcription from a RNA polymerase III promoter it will generate siRNAs with an hairpin structure. Short hairpin RNAs are more stable than synthetic and since siRNAs are continuously expressed within the cells, this method permits long-lasting silencing of your gene of interest.

Further Information: http://www.invivogen.com