Exon 3 was replaced with a modified one in which a point mutation (UGA to UGC) results in the substitution of the catalytically active selenocysteine to a cysteine. A loxP-flanked neomycin resistance cassette was inserted downstream of exon 7. GPX4-specific activity is undetectable in homozygous tissues using PCOOH as substrate. (J:255340)