<n0:ComplexDefinition xmlns:n0="http://ncicb.nci.nih.gov/xml/owl/EVS/ComplexProperties.xsd#"><n0:def-definition>A pathologic process in which alterations at the molecular level result in a more aggressive cytologic and phenotypic profile and clinical course of a neoplasm.</n0:def-definition><n0:def-source>NCI</n0:def-source></n0:ComplexDefinition>
nci:P98
The process of tumor progression is thought to be driven by clonal evolution. The initiating event is a mutation that gives a cell the ability to divide when it shouldn't. As a result a small group of related cells, called a clone because they are identical to each other, grows. Clonal evolution occurs like a tree with mutations causing many clones to branch off continuously.