FIGURE 2.

Evolution of tumor heterogeneity. The graph on the left depicts the evolution of cancer cells as described by the original cancer stem cell theory. Here cancer stem cells (green) can replicate and differentiate into individual clones (red) that over time accumulate mutations secondary to selective pressures (blue and yellow). This results in significant tumor heterogeneity as depicted in the illustration on the right. Tumor heterogeneity increases treatment challenges and nominates a conserved population such as cancer stem cells as a potential therapeutic target.