Figure 1.

Models of CSC formation. In the linear hierarchy model of CSC formation, the transformation events that drive tumorigenesis occur in a stem or progenitor cell that then gives rise to more differentiated progeny as the tumor develops. These differentiated progeny have reduced tumor-forming potential. In the second model, cancer stem cells evolve, perhaps via induction of EMT, either as part of disease progression or in response to selective pressures in the tumor microenvironment.