Predicted cancer risk at different scales: between different species, between individuals from the same population, between organs in an individual, and between cell types (purple cells are more at risk). From top to bottom: the different shapes represent theoretical species and variation in cancer prevalence; the red crosses represent different indivuals of the same species and the number of tumors (e.g. centenarians in a population). The third row illustrates the expected variation of tumor numbers among different organs (e.g. small intestine and large intestine). The last row shows variation in cancer risk at the cellular scale (e.g. stem cells and differentiated cells).