With ES cells, however, scientists learned to make genetic changes not randomly, but in targeted positions in the genome, including within the genes themselves. You could choose to change the insulin gene and—through some rather basic but ingenious experimental manipulations—ensure that only the insulin gene was changed in the cells. And because the gene-modified ES cells could, in principle, generate all the cell types in a full mouse, you could be sure that a mouse with precisely that changed insulin gene would be born. Indeed, if the gene-modified ES cells eventually produced sperm and egg
...more
