What Causes Cystic Fibrosis?

A defect in the CFTR gene causes cystic fibrosis (CF). This gene makes a protein that controls the movement of salt and water in and out of your body's cells. In people who have CF, the gene makes a protein that doesn't work right. This causes thick, sticky mucus and very salty sweat.

Research suggests that the CFTR protein also affects the body in other ways. This may help explain other symptoms and complications of CF.

More than a thousand known defects can affect the CFTR gene. What type of defect you or your child has may influence how severe CF is. Other genes also may play a role in how severe the disease is.

How Is Cystic Fibrosis Inherited?

Every person inherits two CFTR genes—one from each parent. Children who inherit a faulty CFTR gene from each parent will have CF.

Children who inherit a faulty CFTR gene from one parent and a normal CFTR gene from the other parent will be "CF carriers." CF carriers usually have no symptoms of CF and live normal lives. However, carriers can pass the faulty CFTR gene on to their children.

The following image shows how two parents who are both CF carriers can pass the faulty CFTR gene to their children.

Example of an Inheritance Pattern for Cystic Fibrosis

The image shows how CFTR genes are inherited. A person inherits two copies of the CFTR gene—one from each parent. If each parent has a normal CFTR gene and a faulty CFTR gene, each child has a 25 percent chance of inheriting two normal genes; a 50 percent chance of inheriting one normal gene and one faulty gene; and a 25 percent chance of inheriting two faulty genes.