A neomycin selection cassette replaced 300 bp of exon 2/intron 2 including the translation start site. RT-PCR analysis on tooth germ isolated from homozygous mice demonstrated that a aberrant transcript was produced from this allele that spliced exon 1 to exon 3. However, protein analysis and western blot experiments confirmed that no detectable protein was expressed from this allele. (J:71126)