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That being said, it has already been proposed that this technique might be adapted to help some people who cannot produce gametes overcome their infertility. For instance, a fertility specialist could inject bone marrow stem cells from an infertile person into fetal mice in which those human stem cells are transformed into cells that produce human gametes. Such fully human gametes could be harvested from the chimeric mice and used to produce genetically related children by means of conventional in vitro fertilization.
Brownback's final prohibition would forbid the engineering of a non-human life form such that it contains a human brain or a brain derived wholly or predominantly from human neural tissues. Already, Stanford University researcher Irving Weissman has injected human neurons into mouse fetuses producing mice with brains composed of 1 percent human neurons. Weissman next wants to create a strain of mice with brains made almost entirely of human neurons. Such mice would be invaluable for studying human brain diseases and testing medicines to cure those diseases. Mice with brains composed entirely of human brain cells are unlikely to begin contemplating the meaning of life. Why? Among other reasons, because mouse brains weigh just 0.4 grams compared to around 1500 grams for human brains.
Still one can imagine that adding a substantial number of human neurons to fetal primates might end up producing a creature that could be regarded as a diminished human being. However, concerns of this sort should not be allowed to outlaw experiments like that of Yale researcher Gene Redmond. Redmond is trying to find a cure for Parkinson's disease using experiments in which he injects human brain cells into the brains of green vervet monkeys.
This quick review shows that current experiments using chimeric animals and embryos do not threaten our "respect for human dignity and the integrity of the human species." Ultimately, the Human Chimera Prohibition Act is a misbegotten legislative blunderbuss that would criminalize much valuable research aimed at curing human diseases. We can afford to wait until we hear that a Harvard or Stanford institutional review board has approved an experiment to produce a humanzee before Congress needs to act.