pin_gween writes "Scientists have added 100,000 human brain cells to mice in an effort to create realistic models of disorders like Parkinson's Disease. Although mice are 97.5 percent genetically identical to humans and it sounds like a large number, 100,000 only represents 0.1% of the number of cells in mice brains. FTA: 'It's true that there is a huge amount of similarity, but the differences are huge,' Snyder said. 'You will never ever have a little human trapped inside a mouse or monkey's body. [...] Researchers are nevertheless beginning to bump up against what bioethicists call the "yuck factor." 'The worry is if you humanize them too much you cross certain boundaries,' said David Magnus, director of the Stanford Medical Center for Biomedical Ethics. 'But I don't think this research comes even close to that.'"