 by exploiting the ability of retroviruses to move genes into random sites of mammalian genomes and by exploiting some features of their replication , retrovirus vectors have been developed that select for instances in which the virus integrates into expressed genes . since integrated proviruses tag transcriptionally active sites , the vectors 
 provide a means to identify and isolate promoters active in different cell types . 
 furthermore , the viruses may be useful as insertional mutagens , since they select for instances in which integration occurs into expressed sites . 
 this reduces the number of integrants needed to screen for loss of gene function and may enable genes controlling phenotypes in mammalian cells to be isolated.imagesfigure 3.figure 4 . 
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