A neomycin resistance gene replaced exons 2-4. RT-PCR showed that transcripts from mutant animals lacked these exons, and that exon 1 was ligated to exon 5. A stop codon was created by frame shifting in the resulting truncated mRNA transcribed from the direct connection of exon 1 to exon 5, and would result in a 14 amino acid peptide, if any.