81 
For many experiments judged to have a somewhat higher, though 
still only conjectural, potential for hazard, the Guidelines require even 
further attenuation of the K-12 system. Modifications of the properties 
of K-12 and the vectors are required, such that the survival of a genetic 
8 
marker on the vector is less than 1 in 10 (100,000,000) except under 
specially designed and carefully regulated laboratory conditions (Guide- 
lines, Section III- 9). These are the so-called EK2 systems. 
The criteria for certification as an EK2 system have been defined 
and enlarged during the past year, the first full year during which the 
Guidelines have been applicable. Extensive data are required and very 
demanding standards have been set in order that a system be certified 
by the NIH for use as an EK2 system. Design and construction of these 
organisms has been and is being carried out by NIH contractors and 
other interested investigators. Their use in recombinant DNA experi- 
ments is not allowed until they have been certified by the Director, 
NIH, upon recommendation by the Recombinant Advisory Committee. 
Appendix H to this EIS describes the criteria for certification and 
lists the certified EK2 systems. It should be noted that the same depth 
of experience with K-12 that recommends its utility as a host for 
recombinant DNA experiments is central to the ability to manipulate 
K-12 for the purpose of improving its safety. 
c. Use of Simian Virus 40 ( SV40) as carrier of a foreign DNA 
fragment 
It has been argued that the use of SV40 as a vector should not be 
permitted, since this simian virus is known to cause cancer in laboratory 
animals. There is little evidence that SV40 can result in disease in 
humans. However, SV40 does infect humans, as indicated by antibody 
formation, and demonstrable antibodies to SV40 indicate that infection 
has occurred to some extent in the general population. Some of the 
infection may have resulted from the inadvertent inoculation of millions 
of persons during the initial mass immunization against polio virus, 
before SV40 was identified as a contaminant in the vaccine. Or the 
antibodies may have been formed against SV40-like viruses known 
to exist naturally in humans (25). 
