Biomarkers for breast cancer stem cells: the challenges ahead.

Recent studies suggest that a subset of cancer cells with the ability for self-renewal and differentiation into different cell lineages is responsible for tumor progression, metastasis and resistance therapy. These cells, designated as tumor-initiating cells, tumor-propagating cells or cancer stem cells, are of great interest for cancer prognostication and therapeutics. Numerous cell surface and intracellular markers exhibiting cancer stem cell characteristics have been identified in breast cancer, presenting a promise to use them as biomarkers. However, there is a great need for the improvement of experimental methods to detect them in clinical samples, and validate their utility as predictors of the disease outcome, propensity for metastasis and response to treatment. In this article, we present an overview of the current status of breast cancer stem cells, with a focus on biomarkers. We also discuss the technical challenges on the road to defining breast cancer stem cells as biomarkers.