There are two primary types of lung cancer: non-small cell and small cell lung cancer. Non-small cell lung cancer is the most common and comes from epithelial cells and is often described based on the origin. Adenocarcinoma starts in mucus-producing cells. Squamous (or epidermoid) carcinoma begins in the cells that line the airways. These non-small cancers generally spread more slowly. Small cell lung cancer, so called because of the size and shape when seen under a microscope, begins in the nerve cells or hormone-producing cells of the lung and grows and spreads more rapidly. This type of lung cancer is closely linked to smoking habits. It is important to distinguish non-small cell from small cell because the two types of lung cancer are treated in different ways, taking into consideration the stage of the disease.