A PGK-neomycin resistance cassette replaced the 1.2 kb region spanning from within exons 2 to 3 and placed an in-frame translation termination codon immediately after the initiator methionine codon in exon 2, and a putative eukaryotic transcriptional terminator (E. coli tryptophan attenuator) between the inserted stop codon and the neomycin gene. Immunoblot analysis using a rabbit anti-rat antibody did not detect protein in corneal epithelial extracts from homozygous mutant mice. Zymogram analysis of corneal epithelial extracts from homozygous mutant mice did not detect protein activity.