The locus was targeted to have its entire coding region floxed and to express a reporter gene after cre-mediated recombination. A loxP site was inserted in the upstream untranslated region 98 base pairs before the initiator ATG codon. Three repeats of the SV40 early region transcription terminator were added to the untranslated region 340 base pairs downstream of the termination codon, followed by a second loxP site and the coding region of human placental alkaline phosphatase (AP). A frt-flanked neomycin selection cassette followed the AP coding region and was subsequently removed by crossing to mice expressing FLP recombinase. Following cre-mediated recombination, AP activity is detectable in the retina and brain.