The Real Dr John writes: A new study published in the journal Nature provides evidence that intrinsic risk factors contribute only modestly (less than ~10–30% of lifetime risk) to cancer development in humans (abstract). An earlier study had found that the more stem-cell divisions that occurred in a given tissue over a lifetime, the more likely it was to become cancerous. They said that though some cancers clearly had strong outside links – such as liver cancers caused by hepatitis C or lung cancer resulting from smoking – there were others for which the variation was explained mainly by defects in stem-cell division. The new research shows that the correlation between stem-cell division and cancer risk does not distinguish between the effects of internal (genetic) and external (environmental) factors such as chemical toxicity and radiation. They also found that the rates of endogenous mutation accumulation by internal processes are not sufficient to account for the observed cancer risks. The authors conclude that cancer risk is heavily influenced by environmental factors.