U.S. scientists have successfully produced mice with human brain cells, which may be an important breakthrough for developing treatments for brain disorders such as Parkinson's and Alzheimer's, but which may also spark debate regarding the ethics of bioengineering.

Researchers at the California biotechnology company StemCells Inc demonstrated that human brain stem cells can be introduced and grow inside a mouse's skull. The next step, according to Irving Weissman of Stanford University, might be to produce mice with brains made up almost entirely of human cells. However, he stresses that such steps would require a thorough ethical review. "You would want to ask the ethicist what percentage of the brain would be human cells before you start worrying, and if you start worrying, what would you start worrying about?"