4 
Experiments on recombinant DNAs by their very nature lend themselves 
to a third containment mechanism--namely , the application of highly 
specific biological barriers. In fact, natural barriers do exist which 
either limit the infectivity of a vector or vehicle (plasmid, bacterio- 
phage or virus) to specific hosts , or its dissemination and survival in 
the environment. The vectors that provide the means for replication of 
the recombinant DNAs and/or the host cells in which they replicate can be 
genetically designed to decrease by many orders of magnitude the Drobability 
of dissemination of recombinant DNAs outside the laboratory. 
As these three means of containment are complementary , di^ f erent levels 
of containment appropriate for experiments with different recombinants can 
be established by applying different combinations of the physical and 
biological barriers to a constant use of the standard practices. We con- 
sider these categories of containment separately here in order that such 
combinations can be conveniently expressed in the guidelines for research 
on the different kinds of recombinant DNAs (Section III). 
A. Standard practices and training - The first principle of containment 
is a strict adherence to good microbiological practices (4-13). Consequently, 
all personnel directly or indirectly involved in experiments on recombinant 
DNAs must receive adequate instruction. This should include at least train- 
ing in aseptic techniques and instruction in the biology of the organisms 
used in the experiments so that the potential biohazards can be understood 
and appreciated. 
In addition to aseptic experimental techniques, standard practices 
generally include the following procedures when the experiments involve 
organisms that may be pathogenic or may undesirably alter the basic ecology: 
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