Like gene or phlogiston, the term ‘stem cell’ is a scientific concept. Stem cells are very much in the news, thanks to a dramatic upsurge in interest in their therapeutic potential. The recent discovery that stem cell behaviors can be acquired by ordinary cells following the introduction of a small number of genes (reviewed in [2]) has intensified such interest. At the same time, the finding that only a small fraction of the cells within malignant tumors can initiate new tumors upon transplantation has led many cancer biologists to embrace the notion that stem cells are the driving force behind malignancies, and to advocate redirecting cancer therapy toward controlling or eradicating stem cells (reviewed in [3]). Clearly, we live in an era of biology when ideas and theories about stem cells are a major part of the intellectual landscape.