What Information Is available on how much rearrangement or 
recombination with endogenous or other viral sequences is 
likely to occur In the patient's cells? What steps have 
been taken in designing the vector to minimize instability 
or variation? 
A large number of vector constructs have been tested. SAX was designed to 
have as few extraneous sequences as possible and appears to be relatively 
stable. There is minimal homology between the Mo-MuLV sequences remaining in 
SAX and the known human and primate endogenous sequences (see below). 
Therefore, the chances for recombination are low. 
What laboratory studies have been performed to check for 
stability, and what Is the sensitivity of the analyses? 
Detailed analysis of the stability of the SAX vector in vivo has not been 
performed. The one example where the Southern blot was positive on blood and 
marrow obtained from a gene-transplanted monkey revealed a normal-sized 4.9 kb 
band. Therefore, gross rearrangements or deletions were unlikely. CFU-S data 
from the N2 vector suggests that N2 can stably Infect and express, but whether 
or not these data can be applied to the SAX vector (which has a 
primate-promoted human ADA gene and does not express human ADA in CFU-S) is 
currently under investigation. 
(c) What laboratory evidence Is available concerning potential 
harmful effects of the treatment, e.g.. development of 
neoplasia, harmful mutations, regeneration of Infectious 
particles, or Immune responses? 
No laboratory data are available but the theoretical concerns are 
discussed below. Insertion of the SAX vector would confer resistance to 
neomycin and its analogues in the patient's bone marrow cells. However, since 
eucaryotic cells are already resistant to the aminoglycosides used clinically. 
D 
there appears to be no risk from expression of the Neo gene in infected bone 
marrow cells. 
Recombinant DNA Research, Volume 12 
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