To date numerous mouse growth and obesity QTL have been localized [1,2], however, little progress has been made in determining the specific nature of the underlying genetic variants. One resource used to fine map QTL are congenic strains which are designed to convert a complex polygenic trait into one that is mono- or oligogenic. This is accomplished by eliminating segregating genetic variation outside the locus of interest and reducing the environmental variation influencing a trait by characterizing large numbers of genetically identical mice. The Complex Trait Consortium (CTC) considers congenic analysis an excellent method to confirm and subsequently fine map QTL [3].