Methods are provided for identifying a gene at a random chromosomal locus in the genome of a mammalian cell. The method involves inactivating one copy of the gene by integrating one DNA construct (knockout construct) in that gene copy. The knockout construct includes a positive selection marker region sequence and, in a 5' direction from the selection marker region sequence, a transcription initiation region sequence responsive to a transactivation factor, said transcription initiation region oriented for antisense RNA transcription in the direction away from the selection marker region sequence. The second copy of the gene is inactivated by transforming the cells with a second DNA construct (transactivation construct) containing a gene sequence for the transactivation factor which initiates antisense RNA transcription extending from the knockout construct into the chromosomal locus flanking the knockout construct at its 5' end. Inactivation of both gene copies may result in a change in cell phenotype distinguishable from the wild-type phenotype. Optionally, the wild-type phenotype can be regained by introducing a third construct that can inhibit antisense RNA transcription.