The last 21 codons of exon 5 were replaced with a lacZ-neo cassette via homologous recombination and results in an in-frame fusion of the lacZ gene to exon 5. Gene expression was absent in retinas and brain stem of homozygous mutant animals as determined by RT-PCR analysis. Beta-galactosidase activity was not present in the mutant animals. (J:72941)