Clonal expansion, when a single cancer cell explodes into a multitude, is what makes cancers dangerous. “One cell never killed anybody,” says Douglas Brash of Yale Medical School in New Haven, CT. The widely accepted “multiple-hit hypothesis” attributes clonal expansion to a mutation that liberates the cell from normal growth controls.

Suspecting a different mechanism was involved, Brash and his coworkers exposed mice to enough UV light to cause a slight sunburn. After three to five weeks of continuous exposure, the skin of the mice was peppered with expanding clumps of cells that had mutations in the p53 tumor-suppressor gene. However, these clumps stopped growing once the UV lamps were shut off. Because growth required continued exposure to UV, Brash and colleagues concluded that clonal expansion did not involve additional mutations to tumor-suppressor genes. Instead, UV was somehow making the mutated cell's environment conducive to...

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