Exon 3 was disrupted by the insertion of a neomycin selection cassette at the codon corresponding to residue 59 in the mature protein. An RNAse protection assay detected low levels of transcript in homozygous mutant mice. RT-PCR and sequence analyses of the mutant transcript showed that the targeted exon 3 had been excised, resulting in a frameshift mutation that introduces a stop codon after codon 51.