Is it possible that fully differential cells, cells that have acquired specialized functions and perhaps have lost some general properties can change their nature, becoming cells of another type? Professor Okada has studied this problem extensively and introduced the term "transdifferentiation" to describe the phenomenon. The concept of transdifferentiation is extremely controversial, since it challenges a number of biological orthodoxies. The problem is important because if transdifferentiation really does occur, the process of development and differentiation must be much more flexible than...