erythroleukemia (MEL) cells. Analysis of ^-globin mRNA in infected MEL cells 
demonstrated the presence of human specific mRNA which Increased following 
induction via DMSO treatment. In a population of NIH 3T3 cells which were 
infected with the same vector and selected in G418 for Neo R expression, no 
globin RNA could be detected implying some degree of tissue specificity for 
preferential globin expression in MEL cells. The absolute amount of human 
/^-globin mRNA in the induced MEL cells was estimated to be approximately 
500-fold less than the measured levels of mouse ( ^-gl obi n. Similar studies by 
Karsson et al . (1987) using an N2 virus derivative containing the human 
globin gene have resulted in human gene expression at 10% the level of the 
endogenous mouse ^-globin gene. 
In vivo studies. 
The expression of genes in the whole animal by retroviral -mediated 
insertion has been studied primarily by transplantation of vector-infected 
murine bone marrow cells into irradiated mice. The results of several studies 
using different retroviral vectors are summarized in Table 3. One of the 
initial demonstrations of successful gene transfer in mouse hematopoietic tissue 
after transplantation into irradiated recipients was that of Miller et al . 
(1984). A cDNA for human HPRT was inserted into a modified Mo-MuLV vector. 
Expression of the HPRT cDNA in the pLPL vector was controlled by the viral LTR 
sequences. Vector-treated cells were returned to lethally-irradiated mice in 
an attempt to reconstitute the mice with only hematopoietic stem cells 
potentially carrying the introduced DNA and after several weeks from the time 
of transplantation cells of the spleen and bone marrow of reconstituted 
recipients were analyzed for the production of human HPRT. Spleen cell 
extracts from two animals had detectable human HPRT after infection with the 
pLPL vector following separation of human from constitutive mouse HPRT by 
isoelectric focusing. 
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Recombinant DNA Research, Volume 12 
