57 
The risk -assessment standards are specified in detail for one pro- 
karyote host-vector system- -namely, the E. coli K-12 system. The 
Guidelines are organized, for the E. coli systems, according to the source 
and nature of the foreign DNA. For instance, a sample of "foreign" 
DNA containing essentially all the genetic information of an organism 
can be isolated, fragmented, and recombined. If the experiment involves 
such a mixture of DNA fragments, it is referred to as a "shotgun exper- 
iment. " Such experiments are assumed to be of relatively high potential 
hazard (Criterion 1 above) because of the greater likelihood that unknown, 
and therefore possibly hazardous, genes may be introduced into a recipient 
cell, compared with experiments utilizing a single, highly purified 
fragment of DNA. 
Purified fragments containing mainly genes whose properties are 
known and are not harmful offer less potential hazard than a shotgun 
experiment and thus require lower containment. 
In some instances the foreign DNA will itself be derived from extra- 
chromosomal genetic elements. Such extrachromosomal elements 
include the DNA of animal viruses, plant viruses, other eukaryote 
organelles such as mitochondria and chloroplasts, as well as prokaryote 
plasmids or bacteriophages of the same type used as vectors. Each of 
these cases is treated separately in the Guidelines. The prokaryote 
sources are treated differently, depending on whether the source of the 
"foreign" DNA is an organism that does or does not recombine genetic t 
information with E. coli in nature. If the experiment mimics events 
known to occur by natural recombinational processes, then no unique 
hazard can develop from the recombinant DNA experiment (Criterion 9). 
Examples of Containment Systems. Several examples of containment 
systems as combined in practice will be described here. Table I of 
Appendix E shows the containment required for shotgun experiments 
when the foreign DNA is a mixture of fragments derived from eukaryotes. 
The physical and biological containment levels are listed for various 
possible DNA sources: both types of containment must be used, as 
they complement each other. For example, DNA from primates requires 
the most stringent containment, since the estimated potential hazard, 
either from genes that might function in humans with untoward effects 
(Criterion 10) or from pathogenic viral DNAs residing in primate tissue 
(Criterion 2), is judged to be the most serious. Such experiments 
require either P3 and EK3, or P4 and EK2. These alternatives reflect 
the premise that the greater the containment afforded by the host -vector 
system (Criteria 4, 7 and 8), the lower the physical containment required. 
