Division rates can be inferred from cancer genome mutation frequencies and Equation . With a rate of 10-9 mutations per base per division, mutations accumulate with aging in mitotic tissues. Cancer genomes generally have less than one somatic mutation per 100,000 bases (dotted line). A division rate of once every four days rather than once a day is more consistent with observed cancer genome mutation frequencies. In our model, most cancer somatic mutations accumulate earlier in life in normal appearing stem cells (Fig 1).