Based on lessons from the New Hampshire men and a handful of other one-off attempts, the Food and Drug Administration approved pig producer eGenesis to begin a rigorous study of kidney xenotransplants. "Right now we have a bottleneck" in finding enough human organs, said Mass General kidney specialist Dr. Leonardo Riella, who will help lead the new clinical trial. More than 100,000 people are on the U.S. transplant list, most who need a kidney, and thousands die waiting.