Alvin Roth has said that if he could be czar over any one domain, it would likely be kidney exchange. One innovation that he has helped bring about is the non simultaneous extended altruistic donor (NEAD) chain.
In paired donation, where a patient with a willing but incompatible donor is able to receive a kidney from another donor in a similar situation (and vice versa), the surgeries are done simultaneously to prevent donors from reneging. But when an "altruistic donor" makes a non directed donation — that is, to anyone in need — he can set in motion a chain of non-simultaneous transplant surgeries that could, theoretically, go on indefinitely. (The longest NEAD chain to date involved kidney swaps among 60 individuals in 11 states.)
In december 2011, Alexander Berger, '11, MA '11, donated a kidney at UCSF Medical Center. The chain he started is diagrammed below.