Transplanting
organs from
species other than humans (from the
Greek xeno - meaning foreign) into humans. Recent research, for example, has focused on
genetically engineering pigs to have hearts that won't be rejected by humans. This means that these pigs have a small snippet of human gene in their DNA. Pigs turn out to be good candidates because they are roughly the size of humans.
The issue is an ethical minefield; not only because of the perceived grossness of the concept of having a "pig heart" inside you, but also there are significant social risks involved. In particular, many scientists are concerned that in the process of xenotransplantation, a virus that is benign in, say, pigs, will be transferred to humans where it will become an epidemic, killing millions of people.