Chimera mice.
(Picture taken from the NIH website.)

The National Institutes of Health (NIH), primary funder of biomedical research in the US, is reviewing its policy on human-animal chimera research. In light of the abject failure of animal ‘models’ of disease to translate into treatments for human subjects, some researchers are chasing a new holy grail: the possibility of engineering animals with enough human genetic material to make them better models for human pathologies. This research involves appalling pain and suffering for animals, and the prospect of millions more sacrificed on the altar of medical science until such time as this new enterprise is revealed, like much previous animal research, to be a hollow promise.

The NIH is inviting comments on their policy here. For an excellent discussion of human-chimera research and the NIH policy, see this blog posted on the Hastings Bioethics Forum.