"Business Week" quotes Professor Hank Greely, who comments on the ethical debate about placing human brain cells in animals for testing:

In this context the act of swapping a few genes between species is really not much different than breeding a horse with a donkey to create a mule. "There's a bizarre misunderstanding about the nature of life," says Henry T. Greely, a law professor at Stanford who is advising Weissman on the ethics of his mouse experiments. "There aren't 'human genes' and 'goat genes.' There are just genes that have different functions."