2. 外语
  3. Countless people are born with(生来就具有某种特点)the susceptibility to inherit a genetic disease. But scientific progress, especially th

Countless people are born with(生来就具有某种特点)the susceptibility to inherit a genetic disease. But scientific progress, especially th

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问题     Countless people are born with(生来就具有某种特点)the susceptibility to inherit a genetic disease. But scientific progress, especially the art of interfering with(干涉,阻碍) the genetic makeup of the human body, has helped doctors prevent more and more inherited disorders in the last decade.
    Dr. Thomas Caskey of the Baylor University College of medicine in Houston, Texas, is a pioneer in molecular biology(分子生物学). Through the techniques of genetic engineering(基因工程), he transfers genes from one organism to another. Caskey uses a certain type of virus, called a retrovirus(逆转录病毒), as the vehicle for the gene transfer. He first cripples the virus by removing the portion it needs to reproduce itself. The crippled virus becomes harmless while still being able to deliver a cargo to its destination.
    The cargo in Caskey’s experiment is the human A-D-A gene, taken from bone marrow. A-D-A stands for(代表) adenosine deaminase(腺苷脱氨酶), an important component of the human immune system. A defect in the A-D-A gene leads to immune deficiency, rendering(致使) the body defenseless against infections. Caskey’s purpose was to see if the human A-D-A gene could repair the defective immune system of a mouse.
    In the experiment the mouse was given a dose of radiation heavy enough to destroy its immune system. The animal next was injected with the crippled virus carrying the human A-D-A gene. According to Caskey, "the mouse will die within 10 to 14 days unless a successful transfer of bone marrow cells takes place. So we lethally irradiate and subsequently rescue the mouse by bone marrow transplantation(骨髓移植)with the cells that have been infected with the virus." The mouse now carries the human gene that salvaged its immune system.
    Bone marrow transplantation has an established place in contemporary medical practice. Employed to restore the immune system of certain cancer patients and of people who have been exposed to radiation, bone marrow transplantation works only if there is a good match between donor and recipient.
    The procedure would be much easier if bone marrow were like blood. People with type O blood are universal donors(万能供体). Their blood may be transfused to those who have different blood types. Unfortunately, there is no such thing as a universal bone-marrow type. Researchers may have found a way, however, to overcome this problem. The solution, if it works, would be to implant the patient with his own, perfectly matching(型配), bone marrow.
    The idea, as Caskey explains it, is to "correct the patient’s disease with his own ceils, but those cells have added to them a normally functioning gene. "In other words, surgeons would take defective bone-marrow cells from the patient and put them into a laboratory dish where the cells would be exposed to a crippled virus carrying a healthy AD-A gene from a donor. The A-D-A gene would repair the defective cells and then the cells would be reinjected into the patient. Thus, in Caskey words, "the patient would be transplanted by his own ceils containing the added normal gene."
    The technique sounds deceptively(靠不住地) simple. In reality, though(可是,不过, 然而), it is complex. A number of laboratories have tested various intermediate steps of the process, but, according to Caskey, "no single laboratory has put together the entire technology successfully, and highly reproducibly, to proceed with a gene transfer at-tempt in man."
    For some time now, the U.S. National Institute of Health has been taking a close look at(仔细,研究) the effectiveness and safety of the procedure, as well as the ethical questions it raises. There doesn’t seem to be much concern about the ethics of gene transfer into a human being to correct a genetic defect.
    Dr. W. French Anderson of N. I. H. wrote recently that "claims that new organs, designed personalities, master races, or Frankenstein(佛兰肯思泰因，一个创造怪物而自己被它毁灭的医学研究者，英国女作家Mary W.Shelly同名小说中的主角) monsters will be created can be given no credence in the light of(根据,从……来看) what is presently known. "And he added that a well-informed public is the best assurance against any future misuses of genetic engineering.  
What is the most important condition when bone marrow transplantation is used to restore the immune system of certain patients?
选项 A、a good match between donor and recipient
B、the freshness of the bone marrow
C、enough quantity of the bone marrow
D、a universal bone-marrow type
答案A
解析 见第五段和第六段，骨髓移植技术在现代医疗实践中已经成熟。为了恢复某种癌症患者或受辐射人群的免疫系统，在只有供者与受者良好配型时，骨髓移植方可奏效。如果骨髓象血液一样，则这一过程要容易得多。“O”型血的人是万能供体，他们的血可输给其他不同血型的人。但不幸的是并不存在万能型骨髓供体。然而研究人员似已找到了解决这一问题的良方。如果成功的话，患者将可以移植自己的完美配型的骨髓。
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