There's a new theory of how breast cancer works; it's the "on-off switch" theory. Up until now, most researchers have adhered to the "slowly increasing risk" theory, which suggests that the risk of breast cancer is low when a woman is young, and increases with age until menopause, when the risk levels off. But a harder look at the actual cases of breast cancer makes the "on-off switch" hypothesis more plausible. NPR's Joe Palca reports.
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