A protein responsible for repairing damaged DNA may be a vital link to explaining how smoking causes lung cancer, US researchers reported on Tuesday. Lung cells exposed to cigarette smoke produce less of the protein, called FANCD2, the team at Oregon Health & Science University Cancer Institute reported. Without FANCD2, damaged DNA can cause cells to proliferate out of control instead of destroying themselves as normal cells do. The study, published in the British Journal of Cancer, could lead to better treatments for lung cancer, the leading cause of cancer death globally, the researchers said. "These findings show the important role FANCD2 plays in protecting lung cells against cigarette smoke, and may explain why cigarette smoke is so toxic to these cells,' said Laura Hays, who worked on the study.