Our “best" preparations have a helper virus contamination of 0.1%. Murine 
cells Infected with the helper-contaminated S3A viral particles can themselves 
produce Infectious particles. We have no direct evidence whether or not 
infected primate cells are capable of producing infectious particles, but this 
question is under study. Studies on monkeys transplanted with in vitro 
Infected autologous bone marrow cells have never demonstrated infectious 
retroviral particles in their bloodstream (see below). A P3 experiment in 
which monkeys are being injected with large doses of replication-competent 
virus is now underway (see below). We are. of course, attempting to develop 
retroviral vector supernatants that are totally replication-competent virus 
free. 
(b) How stable are the retroviral vector and the resulting 
provirus against loss, rearrangement, recombination, or 
mutation? 
The mutation rate for the SAX vector has not been studied in detail, 
however, tissue culture cells maintained for many passages have remained stable 
(in the presence of G418 selection) when assayed by Southern blot analysis as 
well as by NPT and ADA production. Similar studies need to be carried out in 
the absence of G418 selection. Other researchers have reported that frequent 
point mutations and small deletions can occur during the process of 
transfection and infection. The extent of mutation during a single round of 
Infection (as would occur in a clinical protocol) would probably be small. The 
effect of such mutations on expression of the vector and safety has yet to be 
determined. The literature also suggests that internal promoters (e.g.. SV40 
promoter in SAX) may increase rearrangements, or cause deletions, within the 
vector. We have detected this occurrence in one case in cell lines infected 
with S3A viral particles (see below). 
[46] 
Recombinant DNA Research, Volume 12 
