sequences were absent in the integrated provirus. 
Chang et al . (1987) generated a vector using the pZ IP-NeoSV (X) backbone 
with the cDNA for the human HPRT gene inserted upstream from the Neo^ gene. 
This SVX HPRT B virus was used to infect NIH 3T3 HPRT(-) cells and clones 
expressing the HPRT gene were selected in medium containing hypoxanthine- 
ami nopterin-thymidine (HAT). The level of HPRT activity in ten different 
infected NIH 3T3 cell clones ranged from 4 to 56% of control HPRT(+) cells and 
ranged from 23 to 29% of control values in one of the HPRT(-) human lymphoblast 
clones selected in HAT medium. Isoelectric focusing confirmed that the HPRT 
D 
was the human isozyme. To correlate expression of the Neo gene in infected 
clones initially selected in HAT medium, the relative plating efficiency was 
assessed in G418-containing medium. Colony formation in G418 paralleled colony 
formation In HAT medium with individual clones having a correlation index 
ranging from 0.15 to 0.58. This evidence suggests that the similarity of 
expression of both genes from a single provirus In each clone was most likely 
affected by the site of Insertion or stability of the proviral DNA and was not 
D 
an effect of selection for either gene. In addition, this HPRT/Neo vector was 
used for gene transfer into mouse bone marrow cells i_n vitro . Human HPRT was 
detectable in a pooled population of mouse CFU-C that was initially resistant 
to G418. Unfortunately, the efficiency of gene transfer of this vector was 
much lower in mouse bone marrow cells giving rise to an average of only 0.5% 
G418-resistant CFU-C following Infection. 
Retroviral vectors have been used to introduce intron-containing genomic 
sequences, as in the case of pSVX(RO) carrying a genomic fragment of human 
^-globin cloned into the pZIP-NeoSV(X) retroviral backbone (Cone et al.. 1987). 
The pSVX(RO) vector contains the^?-globin in transcriptional orientation the 
reverse to that of the provirus and was used to Infect both NIH 3T3 and murine 
Recombinant DNA Research, Volume 12 
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