Somewhere right now, a mouse is being born with the express purpose of getting cancer, so someday, maybe, a human won't have to. Somewhere else, a mouse is running in a wheel or splashing through a water maze. A mouse is being made anxious or sleepless or ill. A mouse is being born whose genes have been manipulated to give it diabetes or eliminate its immune system, to make it super-strong or addicted to nicotine.