A DNA fragment encompassing the second and third exons and small segments of the first and third introns has been replaced by a cassette comprising an engrailed 2 splice acceptor (SA) followed by an internal ribosome entry site (IRES), the beta-galactosidase/neomycin fusion gene Betageo, and a polyadenylation signal. Although in homozygous mutant mice, no transcripts containing the deleted exons or exons 10-11 are detected by reverse transcription- (RT-) PCR, transcripts including exons 4-6 and 6-8 are detected; any translation products would be non-functional, lacking portions of the DNA binding and/or coactivator interaction domains.