In summary, we present evidence that breeding Rai1+/− mice in a mixed genetic background results in improved fecundity and expected Mendelian transmission rates compared to the C57Bl/6J congenic strain, providing an additional model in which to study SMS and the role for Rai1 in the development of obesity. The gene discussed is RAI1; the disease is obesity due to melanocortin 4 receptor deficiency.