The endogenous gene was modified in mouse embryonic stem cells by inserting a PGK/neo cassette between exons 3 and 4. LoxP sites were inserted 3' of exon 1 and 5' of exon 4, such that Cre-mediated recombination will remove exons 2 and 3. Exon 3 is crucial for manganese binding and homotetramer formation, and thus recombination is expected to completely inactivate Sod2. (J:226565)