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