Here, the blue stem cell subclone may have a competitive advantage and expand relative to the other stem cell subclones. Thus, the cancer stem cell hypothesis and the somatic evolutionary theory of cancer are not mutually exclusive. Because the non–stem cells are, by definition, not self-renewing, they are evolutionary dead ends. However, the genetic diversity in the non–stem cell compartment should reflect the genetic diversity in the stem cell compartment, with the exception of differences in the proliferation rates of the different non–stem cell subclones, which might slightly skew their frequencies relative to the stem cell pool, and rare genetic alterations that occur during the few cell divisions that separate a non–stem cell from its stem cell ancestor. If neoplastic non–stem cells can revert to a stem cell phenotype, then the evolutionary dynamics become more complex and some of the non–stem cells may not be evolutionary dead ends. The results reported by Park et al. in this issue of the JCI (11) suggest that CD44 and CD24 do not distinguish between stem cells and non–stem cells in breast cancer.