18 
EK2 host-vectors - These are host-vector systems that have been 
genetically constructed and shown to provide a high level of biological 
containment as demonstrated by data from suitable tests performed in 
the laboratory. The genetic modifications of the E_. col i K-12 host 
and/or the plasmid or phage vector should not permit survival of the 
cloned DNA fragment in other than specially designed and carefully 
p 
regulated laboratory environments at a frequency greater than 10“°. 
This absolute measure of biological containment has been selected 
because it is a realistic measurable entity. Indeed, by testing 
the contributions of preexisting and newly introduced genetic pro- 
perties of vectors and hosts, individually or in various combinations, 
it should be possible to substantially indicate, if not prove, that 
the specially designed host-vector system can provide a margin of 
biological containment in excess of that required. 
For EK2 plasmid vectors , no more than one in 10® host cells con- 
taining a chimeric plasmid should be able to perpetuate the cloned DNA 
fragment under nonpermissive conditions designed to represent the natural 
environment either by survival of the original host or as a consequence 
of transmission of the cloned DNA by transformation, transduction or 
conjugation to a host with properties common to those in the natural 
environment. 
In terms of -potential EK2 plasmid-host systems 3 the following 
types of genetic modifications should reduce survival of the cloned 
DNA. The examples given are for illustrative purposes and should 
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