Legend to Fig 4 - Quantitation of Neo containing virions secreted into the 
medium of NIH 3T3 cells transfected with N2 and N4. 
N2 or N4 vector DNA (Fig IB) was transfected into NIH 3T3 cells and productively 
transfected cells were selected with 1 mg/ml G418. Colonies (50-200) were 
pooled, infected with helper M-MuLV at an M.O.I. of 0.01-0.1, and grown for an 
additional 10-15 days until cells became uniformly infected. Infection and 
virus spread was monitored by measuring reverse transcriptase activity in the 
medium. 24-hour-old medium obtained from confluent cells was centrifuged in a 
20-50% sucrose gradient. The visible virus band was removed with a syringe, 
deproteinased, blotted on nitrocellulose paper and hybridized with a -* 2 P 
labelled Neo or M-MuLV LTR specific probe as indicated. 
The Neo and LTR hybridized slot blots were exposed for 24 hours and 2 hours, 
respectively, reflecting the more efficient formation of wild type M-MuLV 
virions. 
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Recombinant DNA Research, Volume 12 
