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Slide 16 
As shown in the next slide (17), there are two animal viruses whose 
DNAs are now technically useful vectors. Cleavage of the DNA of these 
viruses by restriction endonucleases has been studied extensively, and it 
is feasible to insert a piece of foreign DNA into such vectors. Such a 
recombined DNA may now infect appropriate cells, new viral particles are 
produced, and they therefore would contain DNA containing a replica of the 
orginial foreign fragment that was introduced. 
The two viruses are called polyoma and Simian Virus 40, and a few of 
their relevant properties are listed below. In their respective normal 
hosts, mouse for polyoma and rhesus monkeys for Simian Virus 40, neither 
virus causes a known disease. Polyoma does not infect human cells grown 
as single cells in the laboratory, and also does not appear to infect 
humans, since humans exposed to polyoma do not produce antibodies. 
Simian Virus 40 does infect both human cells grown as single cells in 
the laboratory and whole human beings, as evidenced by the act of produc- 
tion of antibodies. This was the virus which was found to contaminate the 
early Salk polio vaccines, and millions of people were inadvertently inoc- 
ulated with it in the middle '50's. To date there is no indication that the 
recipients of the vaccine have suffered any related difficulties. 
Both of these viruses are oncogenic — that is, they cause tumor forma- 
tion in newborn — small laboratory mammals — and both can transform a vari- 
ety of cells of mammalian origin. They are classified as low-risk oncogenic 
viruses by the National Cancer Institute, and the viruses themselves, not 
recombinants, must be handled under conditions equivalent to P2. 
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