Targeting replaced a 0.85-kb segment of the gene containing parts of exons 6 and 7 with a 1.8-kb Pgk-neo cassette having stop codons in all three reading frames and a polyadenylation signal. Immunoblot analysis of microsomal fractions of the cerebra, including hippocampi, of heterozygous and homozygous mice demonstrated reduction and absence, respectively, of detectable RYR3 versus those of wild-type mice. Mice of all three genotypes expressed similar total levels of RYR proteins in their cerebra.