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<b> Purified cellular DNAs other than plasmids, bacteriophages , 
and other viruses 
The formation of DNA recombinants from cellular DNAs that have been 
enriched 5 by physical and chemical techniques (i.e., not by cloning) and 
which are free of harmful genes can be carried out under lower contain- 
ment conditions than used for the corresponding shotgun experiment. In 
general, the containment can be decreased one step in physical containment 
(P4 ->• P3 ■+ P2 PI) while maintaining the biological containment specified 
for the shotgun experiment, or one step in biological containment 
(EK3 EK2 -*■ EK1) while maintaining the specified physical contain- 
ment-provided that the new condition is not less than that specified 
above for characterized clones from shotgun experiments (Section <a>— 
iii). 
<£> Plasmids, bacteriophages, and other viruses 
Recombinants formed between EK-type vectors and other plasmid or 
virus DNAs have in common the potential for acting as double vectors 
because of the replication functions in these DNAs. The containment 
conditions given below apply only to propagation of the DNA recombinants 
5 
A DNA preparation is defined as enriched if the desired DNA represents at least 
99% (w/w) of the total DNA in the preparation. The reason for lowering the con- 
tainment level when this degree of enrichment has been obtained is based on the 
fact that the total number of clones that must be examined to obtain the desired 
clone is markedly reduced. Thus, the probability of cloning a harmful gene 
could, for example, be reduced by more that 10^-fold when a nonrepetitive gene 
from mammals was being sought. Furthermore, the level of purity specified here 
makes it easier to establish that the desired DNA does not contain harmful genes. 
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