The rate of incidence and mortality from a wide variety of common cancers follows a
power law: someone's risk of developing a cancer increases with a power of their age. The model is very simple, and reads \mathrm{rate} = \frac{N p_1 p_2 p_3 \cdots p_r}{(r-1)!} t^{r-1} in Ashley's notation. Their interpretation was that a series of r mutations were required to initiate a tumour. This is now widely accepted, and part of the mainstream view of
carcinogenesis. In their original paper, they found that r was typically between 5 and 7. Other cancers were later discovered to require fewer mutations:
retinoblastoma, typically emerging in early childhood, can emerge from as few as 1 or 2, depending on pre-existing genetic factors. == History ==