animal #10 had a higher proportion of cells containing the integrated vector 
DN A, bat expressed the vector at a much lower level. It appears that vector 
expression in vivo is dependent on variables that are not yet understood. 
Permanent engraftment of cells containing an expressing gene will probably 
require the successful infection of pluripotent self-replicating stem cells and 
a means to maintain in vivo positive selective pressure. Selective pressure 
nay occur for hematopoietic cells containing an expressing ADA gene in patients 
with ADA deficiency (8). 
In conclusion, the data presented indicate that we have been able to 
obtain h-ADA expression in 4 out of 5 reconstituted animals. Four of these 
animals are still alive greater than 9 months after transplant. These aninals 
have shown no sign of retroviremia (S + L~ assays of serum) , marrow dysfunction, 
hematopoietic malignancies, solid tumors or other signs of pathology. The 
animals will continue to be studied. It should be pointed out that the S3A 
cell line produces, in addition to the SAX viral particles, approximately 0.1% 
helper virus. Whether or not this lev level of helper virus affects the 
infection frequency is not established (27) . It is unlikely, however, that an 
ongoing pro- ductive viral infection was established in vivo since no evidence 
of retroviremia has been detected in any of the animals. A complementary gene 
transfer program using the SAX vector into rhesus monkeys has shown low levels 
of h-ADA activity (in 2 out of 3 animals) and neo R activity (in 1 out of 3 
aninals) . In addition, these animals have shown no evidence of retroviremia or 
marrow pathology. Clinical application of this gene transfer protocol will 
require, we believe, a higher and more stable level of expression of the 
inserted h-ADA gene as well as greater reproducibility among animals than has 
been achieved to date. 
Recombinant DNA Research, Volume 12 
[293] 
