Doctors are hailing a new treatment for the lung disease cystic fibrosis as a hugely significant advancement.

A study in the New England Journal of Medicine says the drug called “ivacaftor” targets a faulty protein that causes the disease by correcting the balance of salt and water in the patient's airways.

Those who took the drug had improved lung function and less salt in their perspiration. Salty sweat is one sign of cystic fibrosis.

But the researchers say the drug worked on only about 4 percent of the patients — those who have a specific gene mutation. More than 90 percent of cystic fibrosis patients have a different mutation.

Those with the inherited chronic disease produce a thick mucus that makes it hard to breathe and brings on infections. The mucus can also affect the pancreas. Many patients have a low life expectancy.