Exon 12 was replaced with a mutated version where cysteine codon 462 (TGT) was changed to alanine (GCT) (p.C462A) and a neomycin resistance gene cassette was inserted into intron 11. RT-PCR analysis confirmed the expression of the mutant transcript. This mutation interferes with binding to Mdm2. Immunohistochemical analysis indicates protein expression is increased in homozygous embryos compared to heterozygous or wild-type mice.