Boston.com is running a very readable article on the connections between biochemical and genetic mechanisms of stem cells and cancer. "Researchers know that [the cell signalling pathway called] Notch is active very early in the development of both zebrafish and humans, as the cells of the embryo decide what type of tissues to become. In most cells, it is then turned off. But recently scientists have found that cancer cells can activate the Notch pathway in adults' bodies, causing dangerous changes in their cells. This is apparently what happens in a rare form of leukemia called T-ALL. ... More and more, research is showing that messaging systems used early in development get turned back on and used by helpful stem cells in the adult body, and also by deadly cancer cells."