Balancer chromosomes are genetic reagents that are used in Drosophila 
melanogaster for stock maintenance and mutagenesis screens. Despite their 
utility, balancer chromosomes are rarely used in mice because they are difficult 
to generate using conventional methods. Here we describe the engineering of a 
mouse balancer chromosome with the Cre-loxP recombination system. The chromosome 
features a 24-centiMorgan (cM) inversion between Trp53 (also known as p53) and 
Wnt3 on mouse chromosome 11 that is recessive lethal and dominantly marked with 
a K14-Agouti transgene. When allelic to a wild-type chromosome, the inversion 
suppresses crossing over in the inversion interval, accompanied by elevated 
recombination in the flanking regions. The inversion functions as a balancer 
chromosome because it can be used to maintain a lethal mutation in the inversion 
interval as a self-sustaining trans-heterozygous stock. This strategy can be 
used to generate similar genetic reagents throughout the mouse genome. 
Engineering of visibly marked inversions and deficiencies is an important step 
toward functional analyses of the mouse genome and will facilitate large-scale 
mutagenesis programs.