All but 62 5' bp of exon 3 was replaced with a neomycin selection cassette inserted by homologous recombination. The deleted region encompassed 179 bp of exon 3 and 918 bp of intron 3. Northern blot and RT-PCR analyses showed an absence of wild-type transcript but identified a mutant transcript in which exon 3 was skipped and exons 2 and 4 spliced. Sequence analysis of the aberrant trancscript indicated a frameshift mutation beginning at position 57 and resulting in a stop codon at position 105. Protein was undetected by Western blot analysis using an antibody directed against residues 71 through 92.