12 
make harmful products not be carried out. We differentiate between small - 
and large-scale experiments with such DNAs because the probability of escape 
from containment barriers normally increases with increasing scale. However, 
specific experiments in this category that are of direct societal benefit 
may be excepted from this rule if special biological containment precautions 
and equipment designed for large-scale operations are used, and provided 
that these experiments are expressly approved by the Recombinant DNA Molecule 
Program Advisory Committee of the NIH. 
B. Containment guidelines for permissible experiments - It is an- 
ticipated that most recombinant DNA experiments initiated before these 
guidelines are next reviewed (i.e., within the year) will employ E_. col i 
K-12 host-vector systems. These are also the systems for which we have 
the most experience and knowledge regarding the effectiveness of the 
containment provided by existing hosts and vectors necessary for the 
construction of more effective biological barriers. 
For these reasons, E_. col i K-12 appears to be the system of choice 
at this time, although we have carefully considered arguments that many 
of the potential dangers are compounded by using an organism as intimately 
connected with a man as is £. col i . Thus, while proceeding cautiously 
with col i , serious efforts should be made toward developing alternate 
host-vector systems; this subject is discussed in considerable detail in 
Appendix A. 
We therefore consider DNA recombinants in E_. col i K-12 before pro- 
ceeding to other host-vector systems. 
[ 83 ] 
