Dr. Sell has registered 60 mice that carry either one or two copies of a hypomorphic (partially functioning) mutation in the gene for insulin- like growth factor. Dr. Sell's group derived these mice themselves. Heterozygotes are denoted in the following table as "m/+"; homozygotes are denoted as "m/m". All mice are of the strain 129/CD-1/B6; they are tagged by toe clips and are not being calorically restricted or given any other intervention.