35 
should be chosen that have little or no ability to replicate in 
humah cells. To be used as a vector in a eukaryotic host, a DNA 
moleculfe should display all of the following properties: 
(1) It should not consist of the whole genome of 
any agent that is infectious for humans or that replicates 
to a significant extent in human cells in tissue culture. 
(2) Its functional anatomy should be known— that is, 
there should be a clear idea of the location within the 
molecule of: 
a) the sites at which DNA synthesis originates 
and terminates, 
b) the sites that are cleaved by restriction 
endonucleases , 
c) the template regions for the major gene products. 
(3) It should be well studied genetically. It is desirable 
that mutants be available in adequate number and variety, and 
that quantitative studies of recombination have been performed. 
(4) The recombinant should be defective, that is, its pro- 
pagation as a virus is dependent upon the presence of a comple- 
menting helper genome. This helper should either (a) be integrated 
into the genome of a stable line of host cells (a situation that 
would effectively limit the growth of the vector to that particular 
cell line) or (b) consist of a defective genome or an appropriate 
conditional lethal mutant virus (in which case the experiments 
would be done under non-permissive conditions), making vector and 
helper dependent upon each other for propagation. However, if 
[ 106 ] 
