NEW YORK – Cancer is caused by mistakes in DNA, and a new study finds that in most cancer cases, these mistakes are completely random; they’re not due to heredity or environmental factors, but rather the result of random errors.

The mistakes, or mutations, cause cancer to happen because even a tiny error in DNA can make cells multiply out of control, the study said. Scientists had thought these mutations resulted mainly from two things: Either the mutation was inherited, or it was caused by outside factors that can damage DNA, such as cigarette smoke or ultraviolet radiation, the researchers wrote.

But a third cause — random mistakes — actually accounts for two-thirds of these mutations, said the new study, published today (March 23) in the journal Science.