== 2nd half of opening paragraph has all the right words but not, necessarily, in the right order == 

 The second half of the opening paragraph looks like this: 

  
 Researchers suggest that retroviruses have evolved from a type of transposable gene called a retrotransposon which includes ERVs; these genes can mutate and instead of moving to another location in the genome they can became exogenous/pathogenic. This means that all ERVs may not have originated as an insertion by a retrovirus but rather some may have been the source of origin for the genetic information in the retroviruses they resemble. [4] 

  
 This has lots of nods to interesting and important ERV biology, but is a bit science-salady.  

 Researchers suggest: So I don't care much what researchers may or may not suggest, what matters is what research suggests. Picky point, I know, but scientific authority rests with the research, not the researcher. Suggestion: substitute 'researchers with research. 

 Retrotramsposons aren't a gene. They are, as the retrotransposon page says, genetic elements. They typically are comprised of several genes, together with flanking repeat regions. Suggestion: substitute genes with genetic elements. 

 Stating that ERVs evolved from retrotransposons and then saying that retrotransposons include ERVs seems redundant. It threw me the first few times I read it. Suggestion: just end the sentence after ERVs. 

 The next fragment deals with ERV resurrection, were an endogenous version regains the ability to be exogenous. Does this belong in the introduction at all? Suggestion: replace everything after ERVs;'' with something like this (which needs copy-editing): 

  
 ERVs are passed from parent to child in-place within the genome. Some ERVs retain the ability to 'hop' (transpose) to new positions within the genome[*], and others the ability to make duplicates of themselves[*], allowing ERVs to continually increase their colonisation of the host genome. Occasionally, ERVs pick up mutations that re-activate their ability to function as a retrovirus, becoming the progenitor of a new lineage of retroviruses descended from ERVs[4]. 

  
 We may want references for the two claims I've starred.