population (Kantoff et al., 1986). These results indicate that selection for 
expression of the Neo R gene can enrich for some cells concomittantly expressing 
the introduced human ADA gene. 
Studies of the expression of the SAX vector in whole animals has produced 
mixed results. Evidence in our laboratory for expression of the human ADA or 
Neo R gene from individual spleen foci or pooled foci from reconstituted mice 
has been negative while the expression of the Neo R gene is only 1-102 of the 
Neo R gene in N2 (unpublished observations). On the other hand, the SAX vector 
has been used to introduce the human ADA and bacterial Neo R genes into 
cynomolgus monkey bone marrow cells with subsequent transplantation and 
reconstitution of the irradiated animals. Low levels of human ADA activity 
(approximately 0.01%-0.5% of endogenous monkey ADA) were detected in 4 out of 5 
animals. A complementary study using rhesus monkeys was less successful and 
demonstrated only 0.012 human ADA activity in 2 out of 3 animals studied. The 
two cynomolgus animals which had higher levels of human ADA activity (0.22 and 
0.52) had the elevated levels only between 60 and 120 days post-transplant, and 
there was no evidence of human ADA in either animal at 6 months. Expression of 
D 
the Neo gene in individual peripheral blood mononuclear cells from one of the 
higher expressing animals at 4 months was examined by in situ a nalysis of 
Neo R -specific RNA. Hybridization with the Neo R probe was detected in 0.82 of 
the cells. 
In summary, expression of human ADA in gene-transplanted monkeys is low 
but detectable in some recipient monkeys 2 to 3 months after the time of 
transplantation (1-2 months after full reconstitution) but becomes undetectable 
thereafter. Our inability to detect DNA sequences in most animals in a mixed 
population of peripheral blood cells or bone marrow cells leads us to believe 
that only a few cells carry and express the introduced genes. In contrast to 
Recombinant DNA Research, Volume 12 
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