Exon 2, which encodes the signal peptide and 35 amino acids, was replaced by a lacZ-neo fusion gene. The expression pattern of the beta-galactosidase was shown to resemble that of the endogenous gene via comparisons of X-gal staining and previous in situ hybridization studies. RT-PCR analysis of brain tissue showed an absence of transcript in homozygous mutant mice. (J:33261)