Cells differ from each other 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, observed in the microscope, and detected by biochemical and molecular procedures together comprise what is known as differentiation.

Differentiation resulting 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. It is the business of a cell to produce all of the proteins and enzymes held in common by most cells. The commonly produced gene products are sometimes referred to as housekeeping proteins. However, the adult fly, frog or human are comprised of a great diversity of differentiated cells. The differentiated cells produce, in addition to the housekeeping gene products, tissue-specific proteins. A unique portion...