Cancer cells differ from normal cells in size, structure, function, and growth rate. These malignant cells lack the normal controls of growth seen in healthy cells, and grow uncontrollably. This uncontrolled growth allows the cancer cells to invade adjacent structures and then destroy surrounding tissues and organs. Malignant cells may also metastasize to other areas of the body through the cardiovascular or lymphatic systems. This uncontrolled growth and spread of cancer cells can eventually interfere with one or more of a person's vital organs or functions and possibly lead to death. Cellular growth rates are regulated by proteins produced by the genetic material in cells. Genetic material can be altered or mutated by environmental factors, errors in genetic replication or repair processes, or by tumor viruses. Altered or mutated genes are called oncogenes, and it is these oncogenes that allow uncontrolled growth in cells.