Experimental design of cross-fostering between mice of opposite strains. Breeding pairs of NOD and NOR mice are set up simultaneously. Pups that are born within 48 h of each other to their respective parent are switched to a nursing mother of a different strain. Only half of the litters are switched, leaving half of each original litter with their birth mother. As is standard for the weaning protocols in our animal facility, weaning pups are separated based on sex and nursing mother. Resulting cages will then contain mice of the same sex, but of mixed strains.