Cystic fibrosis and the CFTR protein

Cystic fibrosis is a genetic disease, which means it comes written into the instructions we're born with. Those instructions live in our genes, and the gene that matters here is called CFTR: it's the recipe cells read to build a protein of the same name, the CFTR protein. Each of us has two copies of that gene, one from each parent. Cystic fibrosis appears when both copies carry harmful changes, not just one.

The CFTR protein's job is to move salt and water into and out of cells. When that protein works poorly, the body's mucus ends up without the water it's supposed to have. Think of the difference between a liquid that runs through a pipe on its own and a paste you have to push along: that's what happens to mucus, which turns thick and sticky. That thick, it gets stuck in the passages air travels through when we breathe, in the digestive system that processes food, and in other organs, and it doesn't just clog them: it slowly damages them.