adult mice do not develop lymphomas after MoMLV; it is only newborn mice 
that are susceptible, suggesting some step in T-cell development might be 
important in cellular transformation. Lastly, the tropism (i.e., specificity of 
MoMLV for the T-cell) and the strain dependence of MoMLV are not 
detectably shared with any human cell. 
Nevertheless, it is theoretically possible for a retrovirus to insert into a 
protooncogene or tumor suppressor gene leading to cellular transformation. 
The complex virus-oncogene tropism in the murine system, the presence of 
two copies of each tumor suppressor gene in most cells, and the multistep 
process of malignant transformation suggest that the probability of RMGT 
leading to malignancy in primates is exceedingly low. 
4.5 Recombination with Human Endogenous Retroviral Sequences 
Another theoretical concern is recombination of the retroviral vector with 
human endogenous retroviral sequences, leading to the production of a 
replication-competent human retrovirus. Again, the probability of this 
occurring appears small. No replication-competent human endogenous 
retrovirus has ever been isolated and all known sequences have deletions and 
* frame-shift mutations in the viral genes. The sequences defective in human 
endogenous retroviruses (namely gag, pol, and env), are the sequences deleted 
from the LNL-6 vector, so LNL-6 is unable to provide the needed sequences 
to restore function to human endogenous retroviral sequences. Also, there is 
no homology known between the LNL-6 vector and the human retroviral 
sequences, so that homologous recombination is not expected. In addition, 
variation in the structure of these human and murine viral sequences, such as 
tRNA binding sites, make the likelihood of successful recombination extremely 
small. 
These expectations have been borne out in preclinical and clinical practice. 
Recombination between MLV and human endogenous retroviral sequences 
has not been detectable in primate cells in vitro or in vivo. This question was 
specifically addressed in the LNL-6 TIL human gene transfer clinical trial. 
After retroviral insertion, reverse transcriptase (RT) assays have never 
detected RT activity. 
4.6 Introduction of the neo^ Gene 
The neo^ gene product, neomycin phosphotransferase, phosphorylates the 3’- 
hydroxyl group of the aminohexose I of neomycin and its analogues, thereby 
inactivating the antibiotic. While amikacin may be inactivated by this enzyme, 
gentamicin and tobramycin and other aminoglycosides do not contain a 
Recombinant DNA Research, Volume 14 
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