Insertion of a lacZ-neomycin resistance cassette into exon 1b placed lacZ coding sequences several codons downstream from the signal peptide-coding sequence, and in-frame with Fgf3 coding sequences. This allele expresses beta-galactosidase in a pattern that reflects the expression of wild-type Fgf3 mRNA. RT-PCR analyses of homozygous mutant embryos detected no transcripts.