Lung cancer. Coloured scanning electron micrograph (SEM) of tissue from a lung bronchus affected by adenocarcinoma, cancer arising from the epithelial (lining) tissue. The lining of the lung is brown, cancerous cells are red, and mucus is green. Lung cancers are usually caused by tobacco smoking, and are fatal unless caught early. Magnification: x345 when printed at 10 centimetres wide.