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. Author manuscript; available in PMC: 2009 Jul 6.
Published in final edited form as: J Control Release. 2007 May 13;122(3):385–391. doi: 10.1016/j.jconrel.2007.05.005

Figure 1. The cancer stem cell hypothesis attempts to explain the origin of cancer and identifies the cancer initiating cell as a transformed tissue stem cell.

Figure 1

However, human cancers are heterogeneous and it is not always possible to classify them into stem, progenitor and mature compartments on the basis of differentiation marker expression and morphology. That both stem and progenitor fractions are shown as tumorigenic is derived from our findings in breast cancer (13), as is the bidirectional path between stem and progenitor cells.