to be 10-39% resistant at the time of infection. 20-33% one week after birth, 
and 10-15% after several months. Although standard Southern analysis failed to 
detect vector DNA sequences, the use of PCR has allowed the detection of vector 
sequences in approximately one in 100 to one in 1000 peripheral blood 
mononuclear cells (PBMC). Moreover. NPT activity has also been found in PBMC 
of a treated animal after birth. The finding of a discordance between the 
quantitation of vector DNA and the presence of G418-resistant hematopoietic 
colonies may reflect the variable contribution over time of individual stem 
cells to the progenitor population, or to some other mechanism, e.g.. the 
release of NPT from positive CFU-C under in vitro selective growth conditions 
that protects nearby colonies. 
In summary, these studies demonstrate the utility of antenatal retroviral 
gene transfer. 
V. Studies with Human Hematopoietic Cells; 
Literature review; A Moloney HPRT vector/ amphotropic helper virus 
combination was used to infect a human HPRT-deficient murine lymphoblast line 
(Willis et al.. 1984 and Table Id). Although infection resulted in the 
acquisition of metabolic resistance to HAT medium, cells reverted to HPRT 
deficiency at a rate of 10”® to 10~ 5 events per generation. Reversion was 
associated with either loss, rearrangement, or no alteration of the vector 
sequences. A similar vector containing an HPRT gene coding for a protein with 
altered electrophoretic mobility was used to infect human bone marrow cells 
(Gruber et al.. 1985 and Table Id). Following superinfection with helper virus, 
the vector could be recovered from the bone marrow cells and was shown to 
infect and express in HPRT(-) murine fibroblasts. Chang et al (1987) reported 
the correction of HPRT deficiency in cultured human Lesch-Nyhan cells. 
Frequent vector instability was seen. In a third study. N2 and a spleen 
[30] 
Recombinant DNA Research, Volume 12 
