Like the animals in Animal Farm, we have moved from a position where all cancer cells are equal to one where some cancer cells are more equal than others. There is a specific subset of cancer cells that alone are thought to maintain the growth of the tumour — cancer stem cells.

The identification of these cells is now a crucial goal for cancer research. But how do we discern a true cancer stem cell from a cancer progenitor cell, for example? On pages 903 and 906 we have highlighted two recently published papers that begin to address this question. By building on the advances recently made in the identification of normal adult stem cells, there are a few markers, genes and activated pathways that, when combined with suitable in vitro and in vivo assays, can be used to identify cancer-stem-cell-like cells. However, a definitive set of markers with which to isolate bona fide cancer stem cells within a given tumour type remains to be established.

Accompanying this month's issue is a Poster on 'molecular mechanisms of stem-cell identity and stem-cell fate' by Fiona M. Watt and Kevin Eggan. As this is a relatively new area of research that uses a number of specific terms, the Poster also includes a stem-cell glossary that clarifies some of these. The poster has been produced in conjunction with our sister journal Nature Reviews Molecular Cell Biology and, thanks to the generous support of Abcam, is freely available online at http://www.nature.com/nrc/poster/stemcell.

We hope that this poster will prove to be a useful summary of the origins and fates of stem cells during development and in the adult, and will also provide a guide to some of the markers that have been used to identify stem cells and cancer-stem-cell-enriched populations.