Figure 1.

Origin of Cancer stem cells. In normal tissue, stem cells (green) divide asymmetrically into progenitor cells (orange) from which then terminally differentiated cells (red) are produced (left). In tumorigenesis mutations can transform stem cells into cancer stem cells (light blue) which then result in tumorigenic progenitor cells and differentiated tumor cells (dark blue). But also, by mutations in developmental pathways progenitor cells and differentiated cells can re-acquire stem cell-like properties and turn into cancer stem cells (right).