There is provided a method of identifying DNA responsible for conferring aparticular phenotype in a cell which method comprisesa) constructing a cDNA or genomic library of the DNA of said cell in asuitable vector in an orientation relative to a promoter(s) capableof initiating transcription of said cDNA or DNA to double stranded (ds) RNAupon binding of an appropriate transcription factor to saidpromoter(s), b) introducing said library into one or more of said cellscomprising said transcription factor, and c) identifying and isolatinga particular phenotype of said cell comprising said library and identifyingthe DNA or cDNA fragment from said library responsible forconferring said phenotype. Using this technique it is also possible to assignfunction to a known DNA sequence by a) identifying ahomologues) of said DNA sequence in a cell, b) isolating the relevant DNAhomologue(s) or a fragment thereof from said cell, c) cloningsaid homologue or fragment thereof into an appropriate vector in anorientation relative to a suitable promoters) capable of initiatingtranscription of dsRNA from said DNA homologue or fragment upon binding of anappropriate transcription factor to said promoter(s) andd) introducing said vector into said cell from step a) comprising saidtranscription factor.
