 Depending on the organism and experimental system , the exogenous RNA may be a long strand designed to be cleaved by dicer , or short RNAs designed to serve as siRNA substrates . In most mammalian cells , shorter RNAs are used because long double @-@ stranded RNA molecules induce the mammalian interferon response , a form of innate immunity that reacts <unk> to foreign genetic material . Mouse oocytes and cells from early mouse embryos lack this reaction to exogenous dsRNA and are therefore a common model system for studying mammalian gene @-@ knockdown effects . Specialized laboratory techniques have also been developed to improve the utility of RNAi in mammalian systems by avoiding the direct introduction of siRNA , for example , by stable transfection with a plasmid encoding the appropriate sequence from which siRNAs can be transcribed , or by more elaborate <unk> vector systems allowing the inducible activation or deactivation of transcription , known as conditional RNAi . 
