what is the role of viruses in genetic engineering?

Genetic engineering involves the splicing of genes,
essentially recombining genetic information like the cutting and pasting of text on your
computer.  Genes from different organisms can be incorporated together, for example
allowing livestock to produce human hormones.  This is known as recombinant dna.  But
how do we incorporate these gene splicings into the cells of the host animal?  Well it
turns out that viruses which are snippets of rna code have this ability, it is how they
take over the operations of the cells and turn the mechanisms of the cell into virus
manufacturing centers.  Of particular interest to geneticists are a class of viruses
known as retroviruses which are modified by genetic scientists so they incorporate the
genetic information the geneticists want to implant using the transcription mechanisms
inherent in the virus.