Retroviral particle and genome structure. (A) Retrovirus particle showing approximate location of its components using standardized two-letter nomenclature for retroviral proteins. (B) Genome organization and gene expression pattern of a simple retrovirus showing the structure of an integrated provirus linked to flanking host cellular DNA at the termini of its LTR sequences (U3-R-U5) and the full-length RNA that serves as genomic RNA and as mRNA for translation of the gag and pol open reading frames (ORFs) into polyproteins. env mRNA is generated by splicing and encodes an Env precursor glycoprotein. (LTR) (U3-R-U5) for proviral DNA, derived from R-U5 downstream from 5′ cap and U3-R upstream of 3′ poly(A) in genomic RNA; (PBS) primer binding site; (Ψ) packaging signal; (PPT) polypurine tract; (SD) splice donor site; (SA) splice acceptor site. (Redrawn from Pedersen and Duch 2003, with permission from Wiley.)