DHKR TRANSDUCING RETROVIRUSES 
Amy of helper-free DHFR transducing virus* 
No. or Mu' colonies 
per ari of medium 
No. of helper viruses 
per ml of medium 
PA-LDL1 c6 
1.6 * 10* 
<2 
PA-LDL1 c7 
sj * io* 
<2 
PA-LDL1 cf 
4.2 x 10* 
<2 
* Medium was harvested after 12 b from confluent dishes of the clonal cdl 
Iks and assayed for virus as described in the teat. 
Table 1. (Miller et al., 1985) 
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Recombinant DNA Research, Volume 12 
