An FRT cassette (containing an AUG codon, the lacZ coding sequence and an scs enhancer blocking sequence) separates a 5.2kb EcoRV-BamHI genomic fragment (containing the btd promoter and upstream sequence) from a minigene (hybrid genomic and cDNA sequence) that encodes the 5'UTR and complete coding sequence of btd. In the absence of FLP recombinase, lacZ is expressed under the control of the btd upstream and promoter sequence, but the btd coding sequence is not expressed. Upon excision of the FRT cassette, the btd coding sequence is expressed under the control of the btd upstream and promoter sequence.