Lung cancer is the end-stage of a multi-step carcinogenesis, which starts with a change in the DNA of an alveolar cell. The cancerous cell starts to replicate and spread. Cancer cells are first found in coughed-up mucous, but no tumor can be detected. In the next stage, cancer cells are found in a few layers of lung cells but have not grown to the outer lung surface. Then, the cancer is restricted to the lung and surrounded by healthy tissue, leading to it spreading to the lymph nodes. From there, the cancer spreads to other parts of the body (metastasizes). Treatments include surgical removal, chemotherapy, and/or radiation therapy. In more extreme cases, lung transplants are performed and gene therapy shows much promise.