Reuters has reported an Israeli breakthru in creating functional human kidneys using human embryonic stem cells injected into mice. The stem cell line used in this work was created from human fetuses 7 to 8 weeks old and so are ineligible for follow-on kidney formation work by Federally funded US researchers under President Bush’s recent guidelines. The mice in the experiments formed small human kidneys which were integrated their circulatory system and produced urine. The work was effectively duplicated using pig stem cells to produce small pig kidneys in mice as well. Both types of kidneys did not react with human immune system cells injected into the mice, leading to the conclusion that they could probably be successfully transplanted into humans without rejection due to the presence of pig or mouse proteins. Such xenotransplantation is seen as a possible source of donor organs from animals to humans that could alleviate the shortage of human organs available for transplant. Currently over 50,000 people are awaiting a donor kidney in America alone. Xenotransplantation presents a concern of transmitting viruses across species from animals to humans, however, and is certain to be subject to strict Federal regulation as it develops as a medical therapy.