58 
Another point of interest is that in two instances--primates and 
cold-blooded vertebrates --containment requirements are lower if the 
DNA is isolated from embryonic tissue or germ -line material. This is 
based on the observation that such material is less likely to be contam- 
inated by pathogenic viruses or other adventitious agents than is adult 
tissue. Thus, if the foreign DNA is from adult cold-blooded vertebrates, 
P2 and EK2 are required, but P2 and EKl can be used if the DNA is from 
embryonic or germ -line tissue. 
In some instances, the Guidelines require more stringent conditions 
when the source of foreign DNA is known to be pathogenic or toxigenic, 
or might be infected with a pathogen, or is known to make a harmful 
product. 
Table II of Appendix E summarizes the Guidelines for shotgun exper- 
iments when the source of the DNA is a prokaryotic organism. First 
those prokaryotes that are known to recombine genetic information 
with E. coli in nature are considered. The containment requirements 
are low for this group and vary with the pathogenicity of the source of 
foreign DNA. When this source is a prokaryote that does not naturally 
recombine genetic material with E. coli, the containment recommenda- 
tions are high. It is assumed thaflhe more similar the DNAs of donor 
and host, the greater the probability of expression of foreign DNA. 
In those cases where the donor exchanges DNA with E_. coli in nature, 
it is unlikely that recombination experiments will create new genetic 
combinations never tested by nature. When prokaryote donors not 
known to exchange DNA with E. coli in nature are used, there is a 
greater potential for new genetic combinations to be formed and 
expressed. 
Characterized clones obtained from shotgun experiments may not be 
as potentially hazardous as the original mixture of cells. Cloning of the 
recipient host cell containing the DNA fragment of interest will be one 
of the normal aims of any recombinant experiment. The Guidelines state 
that when a clone has been obtained from a shotgun experiment, and has 
been rigorously characterized, and when there is sufficient evidence that 
it is free of harmful genes, then experiments involving the clone can be 
carried out under PI and EKl conditions if the foreign DNA was from a 
species that recombines genes with E. coli in nature, and under P2 and 
EKl conditions if it does not. Permission for such downgrading of contain- 
ment must be obtained from NIH. 
