Freed by light

Acancer cell can spawn a tumor only if it begins to multiply, a process usually blamed on mutations in oncogenes or tumor-suppressor genes. Now, a new study suggests that carcinogens may promote the growth of cancer cells by altering the cell's surroundings instead of by causing genetic damage.

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...