A gene trap cassette containing an IRESbetageo was introduced into the second intron of the locus. Insertion of the cassette is expected to abrogate the expression of functional protein. The Betageo was flanked with frt sites to allow conditional inactivation of the locus and the frame-shifting exon 3 is flanked with loxP sites. Transcript was not detected by Northern blot of various mutant organs. Protein was detected at 5% of wild-type levels by Western blot analysis, suggesting that the gene trap construct is leaky.