The present invention relates to a method of producing a cell comprising a conditionally active transgene in its genome, the method comprising (a) introducing into the cell a targeting vector, wherein the targeting vector comprises (i) a 5′ recombinase recognition site specifically recognised by a first recombinase, wherein the first recombinase is endogenously present in the cell or wherein the first recombinase or a nucleic acid molecule encoding said first recombinase in expressible form is introduced into the cell followed by (ii) a 5′ recombinase recognition site specifically recognised by a second recombinase, wherein the second recombinase is not endogenously present or is not active in the cell followed by (iii) a selection cassette comprising a positively selectable marker gene followed by (iv) a 3′ recombinase recognition site specifically recognised by a third recombinase, wherein the third recombinase is not endogenously present or is not active in the cell followed by (v) the transgene followed by (vi) a 3′ recombinase recognition site specifically recognised by a fourth recombinase, wherein the fourth recombinase is endogenously present in the cell or wherein the fourth recombinase or a nucleic acid molecule encoding said fourth recombinase in expressible form is introduced into the cell wherein the genome of the cell comprises a 5′ recombinase recognition site and a 3′ recombinase recognition site that are identical to the recombinase recognition sites of (i) and (vi), and wherein said recombinase recognition sites comprised in the genome of the cell are located 3′ of an endogenous cellular promoter such that introduction of the targeting vector into the genome by site specific recombination results in the promoter being operatively linked to the selectable marker gene and (b) culturing the cell in the presence of a selection medium specific for the selectable marker encoded by the selectable marker gene of (iii). The present invention further relates
