Five cloned pigs，whose organs are much l
题型：完形填空
问题：
Five cloned pigs，whose organs are much less likely to be rejected(排斥) by a patient，have been born in the U．S. More than 62,000 people in the U．S. alone are waiting to ____ donated hearts，lungs and so on.The number of human donors falls far short of ____.Pig organs are of a(n) ____ size to human organs，and some scientists hope they might be used to help meet the ____.But previous attempts to transplant pig tissue into humans have ____. The five pigs ____ a gene that adds a sugar to the surface of pig cells.The sugar would ____ immune(免疫的) rejection of the tissue.“This advance provides a neartime ____ for overcoming the problem that there is not enough human organs for transplants，”says an expert.“This is the ____ gene for overcoming the ____ stage of rejection.” ____，scientists warn that much more work is necessary ____ organs from copies of the pigs could be transplanted into humans.Human genes will need to be added，to ____ rejection of the organ in the long term.There are also ____ that pig viruses could infect patients. Cloning techniques were ____ to the production of the pigs.Genes can only be knocked out(去除) in a single cell.Cloning of these single cells then allowed the ____ of a whole animal in which the gene was knocked out in every cell.But the PPL researchers have ____ in knocking out only one copy of the gene.The team will now attempt to knock out both copies of the gene. The team will also ____ tests to investigate whether a virus from the pigs could infect human cells.“Although a lot of the work is very ____，we’re still very far off being able to grow an organ，” says Julia，who is working on this project and quite ____ to creating similar knockout pigs with researchers at the University of Missouri.
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