Lung cancer cells, coloured scanning electron micrograph (SEM). Lung cancer occurs when abnormal cells divide in a chaotic and uncontrolled manner resulting in the formation of a tumour in the lungs. It is more commonly found in older people and one of the most common factors associated with getting lung cancer is smoking. Other causes or risks associated with getting lung cancer are exposure to chemicals or pollutants or a family history of lung cancer. The central cells in this image are displaying membrane ruffling, also known as cell ruffling. This is the formation of actin rich membrane protrusions and often precedes the formation of lamellipodia so is frequently a characteristic feature of actively migrating cells. Cell migration plays an important role in cancer development and metastasis. Magnification: x2000 when printed at 10 cm wide.