A targeting vector was designed to flank exon 2, containing an initiation site and N-terminal 168 amino acids, with loxP sites. Cre expression in the germ line excised exon 2, leaving a single loxP site. Analysis of transcripts suggests that, in the absence of exon 2, exon 1B was replaced by exon 1A, which was spliced to exon 3, thereby allowing translation from an initiation codon in exon 4. The result was a N-terminal truncated transcript. Southern blot confirmed absence of exon 2 in mutant mice with this putatively hypomorphic allele.