MicroRNAs are small molecules that turn down or switch off other genes and influence a wide range of processes in cells throughout the body. Researchers discovered that the microRNA 4423 (miR-4423) is found in higher levels in cells lining the airways of the lungs than in other parts of the body. But, levels of miR-4423 are lower in lung tumors and in otherwise normal-appearing airway cells of people with lung cancer. Because miR-4423 is found on the surface of the airways, measuring miR-4423 levels may serve as a relatively noninvasive test for lung cancer. Adding miR-4423 back inhibited the growth of lung cancer cells in cell cultures and decreased the size of lung cancer tumors implanted into mice. Increasing miR-4423 levels may therefore also form the basis of future lung cancer treatments.