Animals Injected with Human Genes Becoming Commonplace

Inevitably, ethicists are led to debate several questions. Just what makes humans unique? When would a chimera become too human? And, if it did show human physical or behavioral traits, why would that be wrong?
After all, pig heart valves have been used to replace human ones for years. But experiments involving brains, which seem to come much closer to affecting the identity of a man or beast, bring deeper discomfort and more troubling discussions.
Irving Weissman, a researcher at Stanford University, for example, has proposed creating mice with brains containing 100 percent human neurons. By studying such mice, researchers might find cures for diseases such as Parkinson's and Alzheimer's, he reasons.