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that utilizes a case-by-case analysis for determining the 
containment necessary for a particular kind of experiment. 
Number four, identical experiments require 
strikinqly different levels of precautions in different 
nations. Moreover, although the general approach to 
biological and physical containment first proposed in 
the U.S. guidelines has oeen adopted by most other nations, 
the specific characterist ics of each containment level 
differ markedly among different nations. 
Five, in some nations the guidelines specify 
the maximum level of precaution required, and the 
national committee may reduce precautions from this level 
under certain circumstances. In other nations the guide- 
lines specify the minimum level of protection required, 
and precautions may be increased by the actions of local 
or sub-national bodies. 
Six, those guidelines that have been prepared 
more recently are, in general, less restrictive than the 
earlier guidelines. This appears to reflect a changing 
assessment of the risk of recombinant DMA experiments as 
greater experience in this area of research is accumulated. 
However, the guidelines of most nations do not have a tine 
limitation clause, and the frequency of revision of guide- 
lines is usually not specified. 
Seven, the various national guidelines appear 
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