WASHINGTON (AP) -- The first drug that treats the root cause of cystic fibrosis won approval Tuesday, offering a life-changing treatment for a handful of patients with the deadly illness, and broader hope for thousands more patients suffering from the inherited disease.

The Food and Drug approved Vertex Pharmaceutical's Kalydeco to improve lung function in patients with a rare variant of the disease that affects just 1,200 people in the U.S. or about 4 percent of the affected population nationwide. These patients have a protein defect that prevents their cells from properly absorbing and excreting salt and water.

About 30,000 Americans live with cystic fibrosis, a disease that causes sticky mucus buildup in the lungs and other organs, leading to infections, digestive problems and death in young adulthood.