HG is a strain in which the hg deletion has been introgressed onto a B6 background, therefore the only genetic differences between the strains would be the hg locus, tightly linked alleles from the outbred strain on which hg arose and contaminating alleles remaining after the nine backcrosses and fixed during inbreeding. Instead of using parental B6 and HG strains as controls for phenotypic comparisons with each speed congenic, we choose to develop independent control strains originating from the same cross as the congenic panels. Separate B6.CAST control (B6C) and HG.CAST control (HGC) strains were developed using mice from the MMU2 experiment. Mice from the last backcross inheriting only B6 or HG MMU2 alleles at markers spanning MMU2 were intermated to serve as the basis for each control. Both control strains were subsequently maintained through brother-sister mating.