The catastrophic shattering of a chromosome, which can spark cancer development. All or part of a chromosome may break into pieces, some of which reassemble. Leftover fragments can form DNA circles (colored dots, above) that replicate wildly. If the circles contain cancer-causing genes, their proliferation fuels tumor growth. Disintegrating chromosomes may be the cause of many childhood cases of medulloblastoma, an often-deadly brain cancer, researchers in Germany and England report in the March 14 Cell.