As a result of incorrect gene targeting, a neomycin resistance cassette inserted between exons 2 and 3 via homologous recombination. A decrease in gene expression in homozygous mutants was observed by RT-PCR analysis of total midbrain RNA using primers to exons 2 and 12. Western blot analysis of striatal membrane extracts from homozygous mutants showed greater than 95% decrease in protein expression compared to wild-type.