 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 lentiviral vector systems allowing the inducible activation or deactivation of transcription, known as "conditional RNAi".Most functional genomics applications of RNAi in animals have used "C. elegans" and "Drosophila", as these are the common model organisms in which RNAi is most effective."C.