Disclosed is a non-human transgenic animal comprising stably integrated into its genome a first expression cassette and a second expression cassette, wherein said second expression cassette comprises a second polynucleotide encoding a second recombinase under the control of a tissue-specific promoter and a blocking nucleotide sequence, which blocks expression of the second recombinase such that no expression of the second recombinase occurs from said tissue-specific promoter in the non-induced state, and wherein said first expression cassette comprises a first polynucleotide encoding a first recombinase under the control of an inducible promoter, which, upon induction by an inducing agent or composition and expression of the first recombinase, removes the block from the second expression cassette and activates expression of the second recombinase.
