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Abstract

Al-Hajj et al. (2003) were the first to identify and prospectively isolate a minority subpopulation of cells from human solid tumours that contained all of the in vivo tumour-forming abilities. The tumourigenic cell population was identified based on its cell surface phenotype. This population could initiate tumours in immunocompromised mice with as few as 200 cells, while as many as 500 000 or more of the remaining cells in the tumour did not initiate new tumours in mice 1. Fang et al. (2005) have shown that upon culturing of metastatic melanoma cell suspensions under appropriate conditions, a subset of cells could be propagated as non-adherent spheres, which could then be induced to differentiate in vitro and to generate tumours in vivo 2. The ability to acutely isolate and assay subpopulations of cells from tumours that behave as cancer stem cells (CSCs) is essential before performing characterizations such as gene expression profiling, to avoid artifacts introduced by culturing cells for extended periods of time.