Using a homologous recombination strategy, mice were obtained with a modified Mtf1 allele Mtf1loxP where exons 3 and 4, encoding four of the six zinc fingers of the DNA-binding domain, are flanked by loxP sites (Figure 1a). Mice homozygous for the Mtf1loxP allele were further crossed with animals of the Cre recombinase transgenic line Mx-cre. Cre recombinase is expressed in this line under the control of the mouse Mx1 gene promoter, which is inducible by administration of interferon alpha or beta, or synthetic double-stranded RNA pI–pC (32). Cre-mediated deletion was reported to be complete in the liver, while varying in other tissues, ranging from 94% in spleen to 8% in brain (32). After Cre-mediated deletion of exons 3 and 4, which results in a frameshift and premature translation stop, no functional MTF-1 protein can be produced.