A targeting construct was designed to flank the pro encoding region and mature hormone-encoding region of exon V with loxP sites, with a trimerized polyadenylation signal upstream of the 3' loxP sequence and a lacZ gene and FRT-flanked neomycin resistance cassette downstream of the 3' loxP sequence. Cre mediated excision would enable transcription and translation of lacZ in place of Bdnf.