A group of diseases characterized by the abnormal and unrestrained growth of cells in body organs or tissues. Tumour-forming cells develop when the oncogenes (genes controlling cell growth and multiplication) in a cell or cells undergo a series of changes. A small group of abnormal cells develop that divide more rapidly than normal, lack differentiation (they no longer perform their specialized task), and may escape the normal control of hormones and nerves. Cancers differ from benign neoplasms (abnormal growths, such as warts) in that they spread and infiltrate surrounding tissue and may cause blockages, destroy nerves, and erode bone. Cancer cells may also spread via the blood vessels and lymphatic system to form secondary tumours (see metastasis). Causes of cancer include environmental factors such as sunlight, smoking, pollutants, alcohol consumption, and dietary factors. These factors may provoke critical changes in body cells in susceptible people. Susceptibility to certain cancers may be inherited. Many cancers are now curable, usually by combinations of surgery, radiotherapy, and anticancer drugs. For information on particular cancers, refer to the organ in question (for example lung cancer; stomach cancer).