单项选择题

Dolly was once an awfully lonely sheep．When the famous cloned (of an exact copy of a plant or animal made by taking a cell from it and developing it artificially) animal made headlines in 1997，she was the only mammal ever to be manufactured from the cell of an adult donor．Since then，the clone ranks have swelled，with mice and cattle also making their way out of the labs．Last week cloning technology took another step forward when an international biotechnology company announced that it had created a litter of five genetically identical piglets (young pigs)，and that it had a pretty good idea of how they could one day be used：as organ donors for ailing humans．
The idea of turning pigs into tissue factories has been around for at least 30 years．Pigs breed easily and mature quickly，and their organs are roughly the same size as those of humans，meaning operations can be performed with a relative snap-out，snap-in simplicity．The problem is，once the donor organ is stitched in place，the body rebels，rejecting it even more violently than it would a human transplant．“A pig heart transplanted in a person would turn black within minutes．” says David Ayares，a research director with PPL Therapeutics，the biotech firm that helped clone Dolly and also produced the piglets．
What causes pig organs to be rejected so quickly is a sugar molecule on the surface of pig cells that identifies the tissue as unmistakably nonhuman．When the immune system spots this marker，it calls out its defenses．PPL scientists recently succeeded in finding the gene responsible for the sugar and knocking it out of the nucleus of a pig cell．Their next step would be to extract that nucleus，insert it into a pig ovum，and then into the womb of a host pig．The sugar free piglet that was eventually born could then be cloned over and over as a source of safe transplant organs．The idea is to arrive at the ideal animal and repeatedly copy it exactly as it is．The cloned piglets PPL introduced to the world last week were created in just this way，though for this first experiment in pig replication the scientists left the sugar genes intact．
Despite this recent success，PPL is not likely to be setting up its organ shop anytime soon．Knocking out the key sugar gene solves only the problem of short-term rejection．Much more has to be done before any solution to long-term rejection can be rotund．Nonetheless，Ayares is optimistic，insisting that pig organs could be available in as little as five years．For the present，even a little new transplant material is a big improvement over what’s available，and for gravely ill patients awaiting a donor，that’s no small thing． The text is probably written as______．