Helper-Free RrtrtnLnrs Veclon 
constructed which utilize different retrovi- 
ruses for their framework, express genes from 
spliced messages or internal promoters, and 
contain various internal helper virus se- 
quences. At present it is not clear exactly 
which viral sequences are important for gener- 
ation of high-titer, unrearranged vectors. The 
strategy that we have described for rapid 
isolation of vector-producing cell lines should 
facilitate analysis of vector design and the 
production of useful vectors. 
ACKNOWLEDGMENTS 
We thank Dr. Eli Gilboa for providing 
the retroviral vector plasmid pN2. D.R.T. 
would like to thank W. French Anderson, 
Linda Wolff, and Sandra Ruscetti for their 
encouragement and aid in development of the 
DHFR-vector pSDHT. We thank Drs. Max- 
ine Linial, Richard Stead, and Richard Geli- 
nas for suggestions concerning this manu- 
script. This work was supported by a grant 
from the Cooley’s Anemia Foundation. 
A.D.M. is a Fellow of the Leukemia Society 
of America. 
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