indicating that the spliced RNA species is the mRNA for the Neo gene. In SAX, 
but not MAX, infected cells a third RNA species is detected by hybridization 
with a human ADA-specific probe (Fig. 2, Panal D) . This RNA species corresponds 
in size to the SV40-promoted RNA transcript and is the presumptive mRNA for the 
ADA gene product. The absence of a similar transcript in MAX infected cells 
probably reflects the low level of activity of the uninduced MT promoter in the 
infected cells. Fig. 3B shows that the NIH 3T3 cells infected with SAX virus 
contain 2-3 fold more human ADA enzyme activity than endogenous mouse ADA 
activity. On the other hand, MAX infected cells express very low amounts of the 
newly introduced human ADA cDNA, which is consistent with the RNA data shown in 
Fig. 2. 
The N2 vector has been successfully used in diverse experimental systems to 
transfer genes in vitro ( 3 ,4 ,8,9, 10 ,Karlsson et.al. in press; Garver et.al. in 
press) and in vivo (3,9). The unique features of the N2 vector described in 
this report may be responsible for its successful use. 
We thank Dan Kuebbing for DNA sequencing and Evelyn Karsan for the phospho- 
transferase assay. We thank Nancy Plum and Cathy Magruder for assistance with 
the manuscript. This work was funded in part by the American Cancer Society 
grant MV-116C and March of Dimes grant 1-856. 
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Recombinant DNA Research, Volume 12 
