The major cause of death in CF mice is intestinal obstruction, and intestinal obstruction is worsened when the mice are fed standard solid mouse chow [35]. Since the CF mice on the mixed background had better weight gain than on the B6 background and less mucus accumulation in the small intestine, their ability to survive on solid chow was tested. Mice were maintained on the liquid diet until 8 weeks of age, and then switched to solid chow for up to eight weeks. As shown in Fig. 4, wild type mice had 100% survival on chow, as expected. The majority of CF mice on the B6 background died within 2–3 weeks on solid chow, with about 60% mortality over the 8 week period. In contrast, only 22% of the CF mice on the mixed background died (Fig. 4).