announce the discovery of genes responsible for this or that condition, from Alzheimer's to baldness. Research efforts now concentrate on the second and third stages of the process: delivering genes safely to their targets in the body, and controlling gene expression in the altered cells. These steps are crucial to gene therapy's success, and are likely to take the next 10 to 20 years to develop.

Gene therapy developed from the view that disease is a property of genetic structure and regulation. In this perspective, most therapeutic drugs, in effect, act indirectly on some form of gene expression. If ill health is an outcome of faulty cell production—too little or too much of the right proteins—good health is a matter of adjusting the cells' chemical balance.

Much of modern medical treatment depends on the use of chemicals, and a large part of the medical biotech industry involves producing large quantities of pure drugs tailored for specific tasks. Some are extracted from natural sources, some are manufactured synthetic compounds, but more and more are produced by engineering cells with recombinant DNA.

The pharmaceutical business was using the products of cells long before genetic engineering developed in the 1970s. Interest in the potential use of microbes in medicine was stimulated in 1928 by the discovery of penicillin—the first of four major classes of antibiotics now in common use (the others being tetracyclines, cephalosporins, and erythromycins). The original fuzzy mold that settled on some untidy dishes in Alexander