Body mass was recorded for male and female mice at 4 and 8 weeks of age. On the B6 background, female and male mice are about 30% smaller than wild type mice at both 4 and 8 weeks of age (Fig. 1A, B). By comparison, female CF mice on the mixed background were not significantly smaller than wild type mice at 4 weeks of age (Fig. 1A). The improved body weight of female CF mice on the mixed background was maintained at 8 weeks of age. Male CF mice on the mixed background at 4 weeks of age were significantly smaller than wild type and not significantly larger than CF mice congenic on the B6 background (Fig. 1B). By 8 weeks of age, CF males on the mixed background were 12% smaller but this was not significantly different compared to wild type mice (Fig. 1B).