25 
When any investigator has obtained data on the level of contain- 
ment provided by a proposed EK2 system, these should be reported as 
rapidly as possible to permit general awareness and evaluation of 
the safety features of the new system. Investigators are also 
encouraged to make such new safer cloning systems generally avail- 
able to other scientists. NIH should take appropriate steps to aid 
in the distribution of these safer vectors and hosts. 
EK3 host-vectors - These are EK2 systems for which the specified 
containment shown by laboratory tests has been independently confirmed 
by appropriate tests in animals, including humans or primates, and in 
other relevant environments in order to provide additional data to 
validate the levels of containment afforded by the EK2 vectors and hosts. 
Evaluation of the effects of individual or combinations of mutations 
contributing to the biological containment should be performed as a 
means to confirm the degree of safety provided and to further advance 
the technology of developing even safer vectors and hosts. 
2. Classification of experiments using the E. coli K-12 
containment systems 
In the following classification of containment criteria for different 
kinds of recombinant DNAs, the stated levels of physical and biological 
containment are minimums. It is recommended that higher levels of 
biological containment (EK3 > EK2 > EK1 ) be used if they are available 
and are equally appropriate for the purposes of the experiment. 
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