The targeting construct is designed to insert an internal ribosome entry site (IRES), a coding region for a nuclear-localized cre recombinase sequence, and an SV-40 poly(A) sequence, followed by an FRT-flanked neomycin resistance (neo) cassette, between the translational stop codon and transcriptional termination sequence within exon 19. Flp-mediated recombination removed the FRT-flanked neo cassette.