Clonality is a fundamental characteristic of all human cancers. One cancer cell gives rise to daughter cells, all of which exhibit the same change that initially provided a growth advantage to the parent cell. Accumulation of further genetic changes in subsequent daughter cells, each providing an additional growth advantage, has been well documented in human cancer. Correlation of these clonal genetic changes with histopathological progression has led to development of a molecular progression model for colorectal cancer. The identification of genetic markers able to identify the clonal outgrowth of neoplastic cells has proven useful in the detection of a variety of primary neoplasms.