Imperial Cancer Research Fund, Department of Zoology, Oxford, United Kingdom.
An impressive feature of cellular differentiation in metazoa is its stability. This has led to widespread acceptance of the view that the determined state is a heritable property of individual cells. There are, in fact, rather few types of cell for which this has been demonstrated convincingly. Not only is it clear that gene expression is subject to continuous regulation but there is also evidence for an increasing variety of cells that changes in differentiation can be induced by manipulating their environment. Such findings suggest that extrinsic factors may play a more significant role in maintaining the differentiated state of cells than is generally assumed.