The Cre recombinase is under the control of the Mx1 promoter. This promoter is silent in healthy mice, but can be induced to high levels of transcription by administration of interferon alpha, interferon beta, or synthetic double-stranded RNA (such as poly I:C). When combined with a mutant carrying a gene that has been flanked by loxP recognition sites, the expression of Cre recombinase causes the flanked gene to be removed. This provides researchers with the capability to induce the "knockout" at any time during development. There is ~1% background recombination seen in mice not treated with interferon. The percent deletion of the targeted gene varies depending on tissue type, presumably due to the amount of interferon-responsive cells present or to the availability of interferon in each organ.
In an attempt to offer alleles on well-characterized or multiple genetic backgrounds, alleles are frequently moved to a genetic background different from that on which an allele was first characterized. It should be noted that the phenotype could vary from that originally described. We will modify the strain description if necessary as published results become available.
