Abstract
Cellular heterogeneity represents one of the greatest challenges in cancer therapeutics. In many malignancies, this heterogeneity is generated during tumor evolution through a combination of genetic alterations and epigenetic events that recapitulate normal developmental processes including stem cell self-renewal and differentiation. Many, if not most, tumors display similar hierarchal organization, at the apex of which are "stem-like cells" that drive tumor growth, mediate metastasis, and contribute to treatment resistance. Using breast cancer as a model, we discuss how an improved understanding of tumor cellular heterogeneity and plasticity may lead to development of more effective therapeutic strategies.