When working with animal viruses, experiments that involved the linkage of viral genomes or genome segments to prokaryotic vectors and their propagation in prokaryotic cells were to be conducted only with vector-host systems that had demonstrated restricted growth capabilities outside the laboratory and in moderate risk containment facilities. As safer vector-host systems became available, such experiments could be performed in low risk facilities. In experiments designed to introduce or propagate DNA from non-viral or other low risk agents in animal cells, only low risk animal DNA could be used as vectors and the manipulations were to be confined to moderate risk containment facilities.