The authors generated a hypomorphic Rnf2 allele using gene targeting. No genomic sequence was deleted. However, a Neo cassette and 0.8 kb of one of the homology arms were inserted between exons 3 and 4. The inserted sequence was from 3' UTS. Protein levels in the homozygous mutant were 30% of those in the heterozygous mutant. (J:79851)