On the slope of the regression between stem cell divisions and cancer risk, and the lack of correlation between stem cell divisions and environmental factors-associated cancer risk

We are pleased that the analyses on smoking and radiation performed by Little et al. support the idea that the effects of various environmental factors on cancer risk do not correlate significantly with the lifetime number of stem cell divisions. This provides further evidence that the correlation we obtained for the U.S., and similarly found across the rest of the world, is in general not due to environmental factors but rather is due to intrinsic ones. We also point out mathematical and conceptual mistakes in the analysis of Little et al. that led them to erroneous conclusions.