36 
none of these is available, the use of a non-defective genome 
as helper would be acceptable. 
Currently only two viral DNAs can be considered as meeting 
these requirements: these are the genomes of polyoma virus and 
SV40. 
Of these, polyoma virus is highly to be preferred. SV40 
is known to propagate in human cells, both in vivo and in vitro , 
and to infect laboratory personnel, as evidenced by the frequency 
of their conversion to producing SV40 antibodies. Also, SV40 
and related viruses have been found in association with certain 
human neurological and malignant diseases. SV40 shares many 
properties, and gives complementation, with the common human 
papova viruses. By contrast, there is no evidence that polyoma 
infects humans, nor does it replicate to any significant extent 
in human cells in vitro . However, this system still needs to 
be studied more extensively. Appendix B gives further details 
and documentation. 
Taking account of all these factors, it is proposed that: 
1_. Polyoma Virus 
a^ Recombinant DNA molecules consisting of defective 
polyoma virus genomes plus DNA sequences of any non- 
pathogenic organism, including Class 1 viruses (5), can 
be propagated in or used to transform cultured cells 
in P3 conditions; appropriate helper virus can be used 
if needed. Whenever there is a choice, it is urged 
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