The humble house mouse has provided the most important basis for understanding human biology because it has been possible to alter the genetics in mice to order. Mice and men are sufficiently similar for studies with the mice to be able to be related directly to man; thus the laboratory mouse provides the experimental arm for the human genome project. This has become possible through the use of mouse embryonic stem cells. The advent of human embryonic stem cells has opened up ideas for using them as an universal source of therapeutic cells for regenerative medicine.