There is provided a method of identifying DNA responsible for conferring a particular phenotype in a cell which method comprises a) constructing a cDNA or genomic library of the DNA of said cell in a suitable vector in an orientation relative to a promoter(s) capable of initiating transcription of said cDNA or DNA to double stranded (ds) RNA upon binding of an appropriate transcription factor to said promoter(s), b) introducing said library into one or more of said cells comprising said transcription factor, and c) identifying and isolating a particular phenotype of said cell comprising said library and identifying the DNA or cDNA fragment from said library responsible for conferring said phenotype. Using this technique it is also possible to assign function to a known DNA sequence by a) identifying a homologue(s) of said DNA sequence in a cell, b) isolating the relevant DNA homologue(s) or a fragment thereof from said cell, c) cloning said homologue or fragment thereof into an appropriate vector in an orientation relative to a suitable promoter(s) capable of initiating transcription of dsRNA from said DNA homologue or fragment upon binding of an appropriate transcription factor to said promoter(s) and d) introducing said vector into said cell from step a) comprising said transcription factor.
