There will never be enough cadaver organs to fill the growing needs of people dying from organ or tissue failure. This places higher and higher importance, and risk, on living relatives who might serve as donors. Organs that are either redundant (one of a pair of kidneys) or regenerative (bone marrow) become more and more attractive. Transplants become a matter of high- stakes risk-calculation for the donor as well as the recipient, and the intense emotions involved sometimes have people playing long shots.