A single tumor can contain cells in many different states. For example, a tumor might include stem-like cells that can seed new tumors as well as non-stem-like cells. The proportions of these different cell states remain constant over time within a tumor - a finding that has long puzzled scientists. Now, a new mathematical model published in the August 19, 2011 edition of Cell demonstrates that cancer cells are actually able to interconvert between states to maintain equilibrium. This finding brings new light to the effort to understand complexity within populations of cells, and also has implications for improving anticancer therapies.