From the editors | Nature Reviews Cancer

The search term 'cancer stem cells' identifies 31,606 papers in PubMed, a clear illustration, if one was needed, of how popular this area of cancer research currently is. But, what do we

really know about the biology of cancer stem cells? How closely are they related to normal tissue stem cells and does it follow that the cancer stem cell is the cell of origin of the cancer?

Since the identification of leukaemic stem cells, the race has been on to identify similar cells within tumours that arise in solid tissues. This has now been successfully achieved for

several organs, including the brain. However, because various techniques can be used to identify and purify these cells, it is not clear that all cells that are labelled as cancer stem cells

really are cancer stem cells. Angelo Vescovi, Rossella Galli and Brent Reynolds (on page 425) argue that a set of brain tumour stem-cell characteristics needs to be agreed on to enable

researchers to work from the same page. Many argue that cancer arises owing to the transformation of a normal tissue stem cell, so we also require a similar clarification for normal tissue

stem cells. However, as Vescovi and colleagues point out, the cancer stem cell might instead arise from a more differentiated cell, a progenitor cell for example, that has acquired

stem-cell-like functions. So we need to be able to clearly distinguish between the normal and the cancer stem cell.

But this still leaves us with the question of what is the cancer cell of origin, is it the cancer stem cell or does the cancer stem cell arise from the cell of origin? Hopefully, as the

characteristics of normal and cancer stem cells become more clearly defined, the true cellular origin of many cancers will become apparent and so will the cellular characteristics that will

be effective therapeutic targets.

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