A protein responsible for repairing damaged DNA may be a vital link to explaining how smoking causes lung cancer.
Lung cells exposed to cigarette smoke produce less of a protein called FANCD2 based on a US researcher report.
Without FANCD2, damaged DNA can cause cells to proliferate out of control instead of destroying themselves as normal cells do.
The study in the British Journal of Cancer could lead to better treatment for lung cancer, the leading cause of cancer death globally.
Lung cancer kills more than a million people every year and only 15 per cent of sufferers live for five years or longer.