The majority of the exon encoding amino acids 120-176 was replaced with a neomycin resistance cassette via homologous recombination. RT-PCR analysis of liver RNA from homozygous mutant animals revealed a faint truncated transcript resulting from an alternative splice product which skips the disrupted exon. Western blot analysis of brain nuclear extracts detected a shortened protein product expressed at less than 1% the amount of wild-type protein. Co-immunoprecipitation experiments showed that the mutant protein lacked functional capabilities.