In medicine, scientists dream of using cloning to reprogram cells so we can make our own body parts for transplantation. Suppose, for example, you needed a bone-marrow transplant. Some deadly forms of leukemia can be cured completely if doctors destroy your own marrow and replace it with healthy marrow from someone else. But the marrow must be a close genetic match to your own. If not, it will lash out at you and kill you.
Bone marrow is the source of the white blood cells of the immune system.
If you have someone else's marrow, you'll make their white blood cells. And if those cells think you are different from them, they will attack. Today, if you need marrow, you have to hope that a sister, brother, parent, or child happens to have bone-marrow cells that are genetically compatible with your own. The egg would start to divide, to develop, but it would not be permitted to divide more than a few times. Instead, technicians would bathe it in proteins that direct primitive cells, embryo cells, to become marrow cells.