Age dependence of background CML incidence and translocation prevalence. (A) Data⁴⁹  are for background CML during 1973-2008 and for age at diagnosis > 20 years. Data are shown here in a semilogarithmic plot with straight line fits y = Ae^k*^age; female and male k are almost equal. (B) One would expect background chromosomal translocations to correlate with CML incidence because CML is caused by a specific translocation. The data for this plot of cumulative translocation clones in peripheral blood lymphocytes of healthy individuals were obtained (using plotDigitizer) from Figure 4 of Sigurdson et al.⁵¹  Evidence for exponential behavior is not as convincing as in panel A: a significant improvement in fit (P = .015, F or t test) is obtained with an additional a² term, consistent with slight curvature in the data that is visible by inspection. (C) A log-log plot of the same data as in panel B, with the fitted straight line given by y ∝ (age)^1.5. As discussed in “Conclusions,” the translocation results seem inconsistent with a standard assumption in the multistage models usually used to explain age-driven increases in tumor incidence.