When visiting the meat counter of a supermarket, one does not need a microscope to see that liver and steak are different. They are different because their constituent cells are different. Microscopic observation confirms this notion. Liver cells differ from muscle cells in morphology (structure) and this difference is a reflection of physiological activities and biochemical functions that are ultimately under the control of genes. The differences that can be seen grossly (as at the meat counter), observed in the microscope, and detected by biochemical and molecular procedures together comprise what is known as differentiation.

Differentiation results from selective gene action of a genome (the entire genetic complement of an organism) held in common by all cells has been a tenet of modern genetic biology. Certainly the recent cloning of Dolly and other mammals supports this concept. It is the business of a cell to produce all of...