s. le g. and b.k.f . are supported by the intramural research program of the national cancer institute , national institutes of health .
k.j.p . was supported by the homing plus/2012 - 6/12 grant of the foundation for polish science and the national science center , poland ( grant 2011/01/d / nz1/03478 ) .
 replication of retroviruses and transposition of endogenous retroelements exploits a unique mechanism of post - transcriptional regulation as a means of exporting their incompletely - spliced mrnas ( which serve as both the genomic rna and the template for protein synthesis ) . 
 following discovery of the rev response element ( rre ) that mediates nucleocytoplasmic export of the full - length and singly - spliced human immunodeficiency virus type 1 ( hiv-1 ) genome , equivalent cis - acting regulatory elements have been characterized for both complex and simple retroviruses and retroelements , together with the obligate viral and host proteins with which they interact . 
 the exception to this is the gammaretrovirus family of simple retroviruses , exemplified by reticuloendotheliosis virus ( rev ) , murine leukemia virus ( mlv ) and xenotropic mlv - related retrovirus ( xmrv ) . in this commentary 
 , we discuss our recent data that reported structural and functional data on the mlv / xmrv post - transcriptional regulatory element ( designated the pte ) . 
 the pte was characterized by a highly - structured region of multiple stem - loops ( sl1  sl7 ) overlapping the pro and 5 portion of the pol open reading frames , comprising a bipartite export signal whose structures are separated by 1400  nt . in addition , structural probing suggested that sl3 nucleotides were involved in pseudoknot formation . these data , 
 when compared with rna transport elements of complex retroviruses ( hiv ) and simple murine retrotransposons ( musd ) , collectively present an emerging picture that long - range tertiary interactions are critical mediators of their biological function . 
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