Transplanting(移植)organs brings life to the dying. But most donor organs are harvested from the dead. Shortfalls in the number of volunteer donors, the difficulty of gaining the consent of grieving relatives, and a reduction in most countries of the rate of fatal road accidents (the most reliable source of healthy organs), mean that there is a constant lack of them. Thousands die each year while on waiting lists for transplants. Researchers have, therefore, long sought ways to boost supply.
One idea is to harvest animal organs. That is less mad than it sounds. A liver, a kidney(肾脏) or a cornea(眼角膜) does the same job, regardless of species(物种). And it works. In 1984 an American child lived for three weeks after receiving a baboon(狒狒) heart intended as a stopgap(临时替代) until a human donor could be found (unfortunately, one was not found in time). Conversely(相反的), human organs have been transplanted into animals for the purpose of research. Earlier this year, for example, a paper in the American Journal of Transplantation described moving kidneys from human fetuses(胎儿) into rats.
Until now, though, two technical problems have stood in the way of (妨碍)routinely transplanting animal organs into people. One is that the recipient’s immune system (免疫系统)must be persuaded to tolerate a big chunk of (一大块)foreign tissue(人或动植物身体的安排). The other is that swapping(交流) tissues between species risks swapping diseases, too. This second problem may soon be addressed(处理), if George Church of the Harvard Medical School has his way. For, as he and his colleagues describe this week in Science, genetic engineering can now be used to eliminate(消除)one of the most worrying types of pathogen (病菌)that might be spread via transplants.
The animal most commonly suggested as a donor is the pig. Pigs are roughly the size of human beings. They are reasonably well understood. And millennia (几千年)of experience mean they are easy to breed. But they are not perfect. In particular, their DNA is full of retroviruses(反转录病毒，一种致肿瘤病毒), known specifically as porcine endogenous retroviruses(内源性反转录病毒), or PERVS. The genes of these viruses hitch a lift(搭顺风车) from one pig generation to another as an integral part of the porcine genome, whence they can break out and cause infection. And tests in laboratories suggest that, given the opportunity, they can infect human cells as well. (尤其是，猪的DNA中带着大量反转录病毒，切当些是猪的内源性反转录病毒(PERVS)。带着这些病毒的基因随着遗传基因组在猪群中代代相传，在传递进程中就会引起感染。实验研讨表明，只要给时机，这些病毒也可感染人体细胞。)
The existence of PERVs, then, has been one of the main obstacles to transplanting pig organs into people.
Dr Church and his colleagues thought PERVs ideal candidates to test the mettle(勇气;气魄)of one of the rising stars of biotechnology(生物技术), CRISPR/Cas9. This is a gene-editing technique derived from bacteria, which use it as a sort of immune system. In nature, it recognises specific sequences of viral DNA and chops the DNA molecule (分子) apart at these points, protecting the bacterium from harm. Tweaked (稍微调整)a bit in the laboratory, it can be made to recognise any DNA sequence and do likewise. This permits specific stretches of DNA to be deleted(删除) from genomes, and also allows new stretches to be inserted into the gap thus created. (CRISPR/Cas9能辨认带着病毒的DNA序列，在感染病毒的当地将DNA分子与病毒DNA分开以防止病毒的侵害。通过实验稍做调整后，CRISPR/Cas9就能用于辨认各种DNA序列，将特定的DNA序列从基因组中删除，并把新的DNA序列植入由此带来的空缺中。)
Dr Church and his fellow researchers analysed the genetic sequences of one family of PERVs,with a view to (旨在)attacking them with CRISPR/Cas9. They found that the sequence of the gene which lets the virus integrate itself into its host’s DNA is the same from one strain of virus to another. That allowed them to program a CRISPR/Cas9 system to look for this particular sequence and chop it out of the genome. (丘奇博士和他的同事们研讨了一种PERVS基因序列，旨在用CRISPR/Cas9来攻击这些序列。他们发现，能让PERVS与主体的DNA结合的基因序列与使一类病毒与主体的DNA结合的基因序列是相同的。所以他们可以设计出一类CRISPR/Cas9 来辨认这个特定的病毒基因序列并把它从基因组中去掉。)