Note: this site last updated in 2006
An article from "The Molecules of HIV" (c) Dan Stowell
The gag and pol genes are right next to each other in the HIV genome - in fact, they overlap a little - and when the proviral genome is being transcribed from DNA into fresh RNA, sometimes the cell machinery makes a "mistake". Instead of finishing copying out the gag gene, it hops onto the pol gene and also copies that code. So the RNA which comes out is the gag and pol code run together into one longer code.
From the point of view of HIV this isn't a mistake. The combined gag-pol transcript encodes some of its most important proteins, such as reverse transcriptase and integrase. In fact HIV encourages this particular "mistake", with strategically-placed bumps in its DNA to encourage the jump to occur.
As a result, gag-pol gets produced around 1 in every 20 occasions. This means that its resultant proteins are found at around one twentieth of the concentration of the "normal" products.