We watched him on television as he played inside his plastic bubble. We and the rest of the world marvelled that a tiny toddler who had never felt his mother’s skin, or smelled her body or tasted hot food could grow into a bright-eyed, mischievous, highly intelligent and seemingly well-adjusted youth. And then he died. And we mourned, and we asked why.

Young David - the "Bubble-boy" as he came to be known - had SCID: severe combined immune-deficiency disease. SCID is one of an estimated 4,000 human diseases caused by a defective gene, a tiny snippet of one of the 46 long strands of DNA stored in each of our cells. Children born with SCID have a defect in their immune systems; they are born the immunological equivalent of an AIDS patient entering the final stage of disease; They have about the same life expectancy. They usually die of the same causes - mortal infection by microbial pathogens.

Infectious diseases were once the scourge of the human race, felling the majority of people before they even reached reproductive age. That changed with improved public health programs in major population centers, immunization with crippled forms of microbial pathogens that induce immune protection without causing disease, and finally with the discovery of antibiotics. Before the AIDS epidemic, it was rare (although certainly not unknown) for someone to die of an infectious disease in industrialized countries.

But when all of the diseases caused directly by microbial pathogens are accounted for, human beings still find themselves assaulted by a wide range of crippling, even lethal maladies. These are diseases that are idiopathic, arising within us because of some defect in the myriad molecules involved in the construction and operation of the enormously complex human body. A significant portion of these diseases are genetic in origin. Human beings have on the order of 100,000 different genes, instructions written into our DNA for production of the proteins we use to organize and direct every aspect of our biological existence. Defects in the genes encoding any one these proteins can lead to disastrous consequences for the individual inheriting them. The resulting genetic diseases, including such familiar disorders as muscular dystrophy, cystic fibrosis and sickle cell anemia, can be every bit as devastating as infectious disease, and in one way they are much worse: we pass them on to our children, generation after generation after generation.

Having survived the onslaught of microbial pathogens, we now find ourselves prey to an estimated four thousand of these genetically based diseases. Science and medicine have provided us with clues to the treatment of some of them, but by the very nature of these diseases they have never been considered curable. The best that could be imagined is that, knowing the particular protein lacking as a result of a deficient gene, we might be able to administer that protein as a form of medicine. Unfortunately, in all but a very few cases it is simply not possible to deliver the missing protein where it is needed, when it is needed.

All of that is about to change, and it is going to change through one of the most profound revolutions in modern medicine: gene therapy, a branch of the brand new field of molecular medicine. Advances in the laboratory over the past two decades are about to be brought into the clinic; in fact they are already arriving. Progress in the newest field of biology - called molecular biology - have made it possible to isolate human genes, to make billions of copies of them in the laboratory, and to reintroduce them into those unfortunate individuals who have inherited damaged or functionless genes. This same technology turned around on itself can be used to introduce deliberately "bad" genes to attack and destroy unwanted cells, such as cancer cells or cells infected with the AIDS virus.

Molecular medicine, particularly in the form of gene therapy, will be a major part of our lives in the new millenium. The success rate of gene therapy at present is dismal; the number of patients who have been unambiguously cured by it can probably be counted on the fingers of one hand. But the logic behind molecular medicine is powerful and compelling. Everything we know about molecular biology tells us it can work, and that it will work. The federal government and private industry are both betting on it, to the tune of tens of billions of dollars. Over one hundred clinical trials based on this new science are already underway.

Not only doctors, but politicians, attorneys, ethicists and common citizens across all walks of life will find themselves in the position of having to make personal and professional decisions based on a technology that has taken molecular biologists the better part of half a century to develop. In the next decade or two we will have used that technology to "read" the entire human genome. That is easily the medical equivalent of going to the moon - it takes human beings to a completely new level of understanding of our biological selves. We must begin now to think about how we will manage that understanding, and how we will use the information we gain.

Many of us are still recovering from the shock of having computers forced into our lives. But just as computers have changed our lives forever, so too will the tools of molecular medicine, and we will simply have to do what we did with computers: learn to understand them well enough to take advantage of the incredible benefits they can bring us. To do so, we won’t all have to earn Ph.D.s in molecular biology, any more than we had to become mathematicians or electrical engineers to learn how to use computers. The basic elements of molecular biology necessary to understand molecular medicine are easily within the grasp of anyone willing to devote a few hours of thought and reflection. This book will serve as an introduction to this important new frontier of human medicine, and can act as a bridge to the future - a future as exciting and as full of promise as anything we have witnessed in the past century of remarkable progress. Welcome aboard!