Abstract: A method of expressing RNAi in cells including introducing into cells a molecule of nucleic acid including the sequence sense and antisense of RNAi placed under the control of a promoter of single transcription, which sense and antisense sequences are separated by a sequence of DNA including a sequence for the stop of this transcription, which DNA sequence is framed at each of its ends by a lox site, and (ii) placing Cre in contact of the lox sites to obtain by site-specific recombination the elimination of the DNA sequence and the stop sequence of the transcription such that the sense and antisense sequences are no longer separated except by the remaining lox sequence and, thus, permit transcription of the RNAi in its entirety with the residual lox sequence as a loop.