the evolution of viruses with expanded host ranges. \JJ 
Lear discusses a case in which the combination of a baboon 
virus and a mouse tumor virus resulted in a hybrid with the 
unexpected ability to cause tumors in mice, dogs, chimpanzees, 
and baboons, and also to infect human cells in culture.—/ 
The reduction in containment levels mandated for the cloning 
of viral DNA using eukaryotic viral vectors and viral 
complimentary DNA in prokaryotic host-vector systems is, 
therefore, a bold step. (See Tables III and IV, pp. 33079 and 
33083.) The stipulation that some experiments will be 
assigned containment levels on a case by case basis ignores the 
need for minimum standards which can serve as a guide for 
research workers. 
We therefore strongly recommend that specific biological 
and physical containment levels be assigned for experiments with 
hybrid viruses. In addition, caution should be exercised 
when productive viruses are used as vectors for recombinant 
DNA experiments. 
G. Shotgun Experiments 
The proposed reductions in containment levels mandated 
for shotgun experiments are quite substantial. They are based 
on a new confidence in the integrity of host-vector systems , 
the safety of cloning complete viral genomes, and the necessity 
17 / M. M. Kaplan and R. G. Webster, Scientific American, 237 
(1977), 88-106. 
18/ J. Lear, Recombinant DNA: The Untold Story (New York: Crown, 
1978), 253-254. 
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