Schematic overview of the two main hypotheses of breast cancer stem cell hierarchy. (A) Cancer stem cells irreversibly convert into to progenitor cells. The cancer stem cells have the possibility of asymmetrical division, producing a new cancer stem cell and a tumor cell, whereas differentiated tumor cells do not. Changing colors represent the introduction of somatic events in the tumor. Arrows between stem-like cancer cells denote a slow turnover rate. The much higher turnover rate of differentiated tumor cells means that mutations that arise in those cells by far would outnumber the ones that originate in stem cells. (B) Dynamic state between cancer stem cells and differentiated tumor cells, where interconversions between differentiation-states is possible. The rapid turnover of differentiated tumor cells yields a spread of mutations across both the bulk tumor and cancer stem cells.