mately aim to give us a complete molecular inventory of ourselves.

The first successful organ transplant was carried out in 1951, when a kidney survived the move from one body to another and kept working. In the mid-1990s, kidney transplants have become almost a commonplace triumph of surgery, with about 100,000 defective kidneys being replaced around the world every year. Several former patients have already lived more than 20 years with a kidney they were not born with.

Liver and lung transplants were first attempted in the 1960s, but no operation caught the public's attention more than the first heart transplant, carried out by South African surgeon Dr. Christian Barnard on December 3, 1967. Like the first human in space, the pioneering physician made science into headlines and became a household name. Like the space program in the '90s, heart transplants now merit barely a paragraph on an inside page of the newspaper. In the United States today, where most of these operations take place, more than 2,000 hearts each year are set beating inside new chests.

One of the key discoveries that helped make organ transplants possible in the first place was the concept of matching tissues. Unless the tissue of a donor matched that of the recipient very closely, a transplant was likely to fail. The patient's body, able to distinguish between the tissues of "self" and "non-self," would reject the new graft as an unwanted invader.

British researchers Frank Burnet and Peter Medawar first realized the importance of matching tissues during the 1940s after studying how the immune system works. They found that the body responds to invading foreign materials, such as viruses, by producing white blood cells