Approximately 2330 base pairs of 5' flanking sequence from the rat airway-specific Scgb1a1 (also called the Clara cell secretory protein) gene were inserted into the pTG1 vector upstream of exon 1 (104 base pairs, including the transcription initiation site), intron 1 (947 base pairs) and 10 base pairs of exon 2 from the mouse transthyretin gene; the protein-coding region of the mouse Scnn1b cDNA, inserted just downstream of the transthyretin exon 2 splice acceptor, is followed by the SV40 early-region polyadenylation signal. In situ hybridization with a transgene-specific antisense probe confirmed expression in airway epithelia of the lung. The insertion site was identified by in situ hybridization; it is located on chromosome 5 near the B-C1 chromosome band junction.