A hallmark of cystic fibrosis, a disease caused by mutations in the CTFR gene, is the accumulation of abnormally thick and sticky mucus in the lung, intestine, and various other organs. Although the accumulation of this mucus is thought likely to play a central role in the development of disease, how mutations in the CTFR gene lead to mucus accumulation have not been determined. Scientists have now provided insight into this issue by studying mouse small intestine segments ex vivo.