An frt flanked neo cassette followed by a loxP site were inserted upstream and a second loxP site was inserted downstream of a cluster of 7 dioxin responsive elements (DRE). These DREs regulate expression of both Cyp1a1 and Cyp1a2. The neo cassette and DRE cluster were then removed by Flp and Cre mediated recombination respectively. (J:147150)