Expression of embryonic characters by malignant cells

While the expression of fetal characters by malignant cells is now well-documented, the mechanism involved and the nature of the cells that are the target of malignant transformation remain controversial. It has often been assumed that carcinogenesis results in 'dedifferentiation' of specialized cells. The alternative hypothesis is that neoplasia results from a disorder of some normal stem cells. This view is discussed in relation to several examples, in particular teratocarcinomas.