oujciy uj ici/o virus gcuc incrupy, i cnun, 
other changes needed in a cell with an activated proto-oncogene to give a malignancy, 
since insertional activation of any one proto-oncogene is apparently not sufficient for in- 
duction of cancer (Temin, 1986a). 
The removal of most U3 sequences in the vector so that there are no potential 
retrovirus activating sequences in the infected cells (Figure 2) reduces the probability of 
insertional activation of a proto-oncogene by integration of the vector (Yu et al.,1986; 
Dougherty and Temin, 1986b). The effectiveness of proto-oncogene activation by control 
sequences remaining in the vector for expression of the inserted gene will have to be 
determined experimentally. However, as discussed above, even if the final probability is 
significant, it should not be a barrier to this therapy for life-threatening conditions. 
Mutations in the vector will occur at a high rate as with any retrovirus during 
replication (Coffin et al.,1980;1986; Dougherty and Temin, 1986a). In most cases these 
mutations inactivate the vector or the gene(s) it carries. Such mutations are not 
dangerous. They merely reduce the efficiency of the therapy. However, it is possible that 
some mutations could increase the ability of residual control sequences to activate proto- 
oncogenes or result in the infected cells producing a product toxic for the organism, but 
not for the infected cell. The probability of such mutations must be very low. 
B. Theoretically Possible, But Unlikely Biological Problems 
1. formation of a replication-competent retrovirus 
The possibility of the recombination of a retrovirus vector with the genomes of 
the infected cells, the helper cells, or another virus to form infectious virus seems to be of 
most concern to the general public. The probability of formation of an infectious virus 
depends upon the amount of nucleotide sequence homology between the vector and the 
helper cell and between the vector and human cells (Anderson et al.,1984; Bandyopad- 
hyay et al.,1984). In the helper cell, both sequences expressing viral proteins and other 
Recombinant DNA Research, Volume 12 
[ 382 ] 
