PREVENTABLE WORRY. “Some scientists and bioethicists fear the stem cells could create human eggs and sperm in the animals. To say the least, it’s disconcerting to think about two mice making a human embryo…scientists could engineer the cells so that they were unable to form human brain cells, sperm or eggs. The animals could also be isolated or sterilized to prevent them from breeding.”
http://www.npr.org/sections/health-shots/2015/11/06/454693391/should-human-stem-cells-be-used-to-make-partly-human-chimeras