5 weeks safety report, page-4

"...For longer-lasting silencing, short hairpin RNAs (shRNAs) are the molecules of choice...."

".......more-sustained knockdown might be required .......................shRNA can produce steady-state levels of siRNA indefinitely because the viruses that deliver the shRNAs are typically retroviruses, specifically lentiviruses, mainly derived from HIV...."

http://www.the-scientist.com/?articles.view/articleNo/36692/title/Gene-Silencing-Is-Golden/

end

old article - but good for newbies