There was a period in the history of modern biology when proteins were thought to be gene products in a rather direct sense. An account of their appearance and disappearance in the course of development and differentiation thus seemed an appropriate means of describing gene regulation. When RNA was found to be the immediate product of genetic activity, the study of proteins as gene products lost some of its original attraction. Indeed, the development of the powerful method of nucleic acid hybridization aroused the hope that a large array of specific messenger-RNA molecules synthesized during...