The beta-geo gene trap was inserted into intron 23. This mutation creates a fusion of all three isoforms that disrupts the Rho-guanine nucleotide exchange factor (GEF) and protein kinase D (PKD) binding domains. Quantitative PCR analysis of neonatal heart and lung RNA revealed that expression of the E37-38 exon-exon junction is reduced in heterozygous and completely eliminated in homozygous mutant mice, while expression of the E4-5 exon-exon junction remains unaffected.