Both of these questions are surprisingly thorny. Consider the matter of potency: the usual definition is the ability to generate cells of other types, but there are two problems with this. First, there are clearly cells in the body that generate cell types other than themselves but which are not regarded as stem cells (for example, those that are often called ‘committed progenitors’). We can distinguish such cells from stem cells by requiring the latter to be at least bi-potent (able to produce at least two cell types other than themselves), but it is not clear that adult stem cells necessarily have this property. And if we intend to include cancer stem cells in our definition, we would be hard pressed to find any good reason to demand that those cells produce more than one type of differentiated offspring. A converse issue arises during embryonic development, because the vast majority of cells, at early stages at least, go through many sequential multipotent stages. Unless we wish to call all of them stem cells (which would seem to rob the concept of much of its utility), we are forced to conclude that high potency alone is an insufficiently restrictive criterion by which to define stem cells.