Safety of retrovirus gene therapy, Temin, 
sequences in the cellular genome are relevant. As discussed above, vectors can be con- 
structed so that there is no homology between the vector and helper cell nucleotide se- 
quences, and vectors can be constructed or selected so that there is no homology between 
vector and human cell nucleotide sequences (Kwok et al.,1986). Thus, the formation of 
an infectious virus would depend solely upon non-homologous recombination (see 
below). Furthermore, as discussed above (Figure 1), the safest vector would contain only 
R-U5-arrR-PBS-E-insert(s)-PPT-tffrL-R. Therefore, the endogenous sequences involved 
in such recombination would have to contain all of the other sequences of a replication- 
competent retrovirus. Since these minimal vector sequences are less than 10% of the 
genome of a replication-competent retrovirus, it is unlikely that such an otherwise 
replication-competent retrovirus exists. However, in no case is the sequence of the 
human or other mammalian genome known in enough detail to establish that such a 
replication-defective provirus does not exist. 
For discussion, assume that such replication-defective endogenous viruses exist. 
Then what is the possibility of their non-homologous recombination with a minimal 
retrovirus vector to give a replication-competent retrovirus? The probability of non- 
homologous recombination of transfected DNA with endogenous sequences to give a 
replication-competent retrovirus when there is some homology elsewhere between the se- 
quences is between 10'^ and 10*2 (Bandyopadhyay et al, 1984). The probability of non- 
homologous recombination when there is no homology between the sequences must be 
considerably lower. Moreover, transfected or infected helper cells can be screened for the 
occurrence of such recombination by cloning the helper cells and assaying each clone for 
the presence of infectious virus (Miller et al., 1986). 
Non-homologous recombination could also occur after infection of human bone 
marrow cells with the vector. The existence of highly oncogenic retroviruses indicates 
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