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Normal cells will divide in the presence of a chemical signal such as "growth factor" (GF). Other molecular signals from within cells or from their environment can cause cells to halt division. In healthy tissue, damaged cells are eliminated by programmed cell death (apoptosis) and are replaced when neighboring cells undergo mitosis. A mutagen, such as UV light, may lead to mutations in cellular DNA. Mutated cells with damage in critical areas of their DNA can undergo mitosis whether or not growth factor is present.