Lung cancer, also known as lung carcinoma, is a malignant tumor that begins in the lung. Lung cancers are caused by genetic damage to the DNA of cells in the airways, often exacerbated by cigarette smoking, or inhaling damaging chemicals. Damaged airway cells sometimes gain the ability to proliferate unchecked, causing the growth of a tumor. Without treatment, lung tumors can spread throughout the lung, damaging lung function. Eventually lung tumors metastasize, spreading to distant parts of the body, and causing varying disease. Lung cancers are classified based on the cells they originate from. Around 15% are small-cell lung cancers, while the remaining 85% (the non-small-cell lung cancers) are adenocarcinomas, squamous-cell carcinomas, and large-cell carcinomas.