Exons 2 - 4 were flanked with wild-type and mutant loxP sites, an inverted IRES-lacZ cassette was inserted between exons 4 and 5 inside the loxP sites, and an FRT flanked neo cassette was inserted between exons 4 and 5 outside the loxP sites via homologous recombination. Flp mediated recombination removed the neo cassette. Cre mediated recombination inverted the IRES-lacZ cassette and removed exons 2 -4.