The RIZ1 protein product was disrupted by insertion of a neomycin resistance cassette into the center of the sequence encoding the PR domain in exon 5. The RIZ2 protein product, which begins transcription at exon 6, was not affected by the gene targeting event. Absence of RIZ1 gene expression in homozygous mutant animals was verified by Northern blot of adult mouse brain RNA, Western blot of adult testis proteins, and RT-PCR analysis.