Mice and pigs are easy; rats are harder. Humans are harder still, but not as hard as monkeys. No one knows why some species are tougher to clone than others. But studies with mice are helping define those differences and should ultimately enable researchers to achieve greater success in obtaining human embryonic stem cells to treat diseases and replace organs.
In terms of therapeutic cloning (otherwise known as nuclear transfer), the most notable achievement so far comes from Woo Suk Hwang of Seoul National University. In February his lab reported on how it transferred the nuclei of several human donor cells into 242 eggs that had their nuclei removed. The batch produced 30 blastocysts (an early stage of embryonic development), of which only one yielded a self-renewing stem cell line.