LoxP sites were inserted flanking exon 5. An adjacent frt-flanked neomycin cassette was removed via Flp-mediated recombination in zygotes. An alkaline phosphatase gene was inserted just 5' to the second lox P site such that upon cre-mediated recombination, the alkaline phosphatase gene is expressed under the control of the Fgf8 gene. (J:66265)