Multistep carcinogenesis through sequential cycles of mutation and clonal succession is usually described as tumor progression, or the clonal evolution of tumor cell populations. However, many mutations found in cancers are also compatible with normal appearing phenotypes and therefore genetic progression may precede tumor progression. To better characterize such pretumor progression (mutations in the absence of visible phenotypic changes), a quantitative model was developed that postulates most oncogenic cancer mutations first accumulate in normal appearing colon crypt niche stem cells