For decades, public health officials have puzzled over a surprising fact about HIV: only about 10-20 percent of infants who are breastfed by infected mothers catch the virus. Tests show, though, that HIV is indeed present in breast milk, so these children are exposed to the virus multiple times daily for the first several months of their lives.Now, a group of scientists and doctors from Duke University have figured out why these babies don’t get infected. Human breast milk naturally contains a protein called Tenascin C that neutralizes HIV and, in most cases, prevents it from being passed from mother to child. Eventually, they say, the protein could potentially be valuable as an HIV-fighting tool for both infants and adults that are either HIV-positive or at risk of contracting the infection.The research, published in Proceedings of the National Academy of Sciences, was inspired by previous work done by other researchers showing that, both in tissue cultures(组织培养) and live mice, breast milk from HIV-negative mothers was naturally endowed with HIV-fighting properties. Scientists suggested that a few different proteins in the milk could potentially be responsible, but no one knew which one.As part of the study, the researchers divided breast milk into smaller fractions(部分) made up of specific proteins via a number of filters(过滤器) — separating the proteins by size, electrical charge and other characteristics — and tested which of these fractions, when added to a tissue culture, prevented the cells from being infected by HIV. Eventually, they found that one particular protein was present in all the HIV-resistant fractions but in none of the others: Tenascin CTenascin C works by blocking a key protein on HIV’s envelope that normally ties up to a receptor on a T cell’s membrane called CCR5. In doing so, Tenascin C prevents HIV from mixing with the T cell and injecting its RNA inside.Still, the researchers say that other natural elements in milk might play a role in fighting HIV as well. "It’s clearly not the whole story, because we do have samples that have low amounts of this protein but still have HIV-neutralizing activity," the study’s lead author Permar says. "So it may be acting in concert with other antiviral and antimicrobial factors in the milk."Whatever those other factors are, though, the finding vindicates (证明) recent changes to UN guidelines that recommend even HIV-positive mothers in resource-poor countries should breastfeed, if they’re taking anti-retroviral drugs to combat their own infection.The next steps, Permar says, are determining which area of Tenascin C is active and whether it can effectively prevent transmission in a live animal. If it works, it could potentially be incorporated(并入) into an HIV drug with broader applications. It’s even possible that it could someday be adapted to reduce the risk of HIV transmission in adults as well as infants.1.What can we know about Permar and the research group according to the passage?A.They have found a cure for HIVB.They have helped change UN guidelinesC.They will do more researches on Tenascin CD.Their finding is based on their previous work2.What is the main idea of Paragraph 1?A.The breast milk from an infected mother doesn’t contain HIV at allB.Only a small percentage of infants breastfed by infected mothers catches HIVC.The possibilities of infants catching HIV have nothing to do with infected mothersD.Children feeding on infected mothers’ milk won’t catch HIV though exposed to it often3.Why did the researchers divide breast milk into smaller fractions?A.To find out Tenascin CB.To make up specific proteinsC.To add them to a tissue cultureD.To test what prevents cells from being infected4.Which of the following about Tenascin C is true?A.It has become an effective tool in fighting HIVB.It is the key protein on HIV’s envelopeC.It can prevent transmission in a live animal effectivelyD.It helps prevent mother-to-child transmission of HIV