Thetargeting vector was constructed to insert a construct directly after the ATG start codon of the mouse Atf3 gene containing: Cre fused to the mutated estrogen-ligand binding domain (CreERT2), 1.2 kb of Atf3 3' untranslated region, a Simian virus 40 stop signal and a neomycin cassette flanked by two FLP recognition target sites. The neo cassette was removed from founders by crossing with Flp recombinase mice. It is a null allele.