Structure of proviral PERV Structure of proviral PERV. (A) Genes and open reading frames are shown as open boxes. cap, transcriptional start site; PBS, primer binding site; SD, splice donor; SA, splice acceptor; SU/TM, surface/transmembrane envelope protein cleavage site in Env; PPT, polypurine tract, poly(A) addition site; LTR, long terminal repeat; gag, group-specific antigen gene; ppro/pol, protease/polymerase gene; env, envelope protein gene. Structure of proviral PERV. (A) Genes and open reading frames are shown as open boxes. cap, transcriptional start site; PBS, primer binding site; SD, splice donor; SA, splice acceptor; SU/TM, surface/transmembrane envelope protein cleavage site in Env; PPT, polypurine tract, poly(A) addition site; LTR, long terminal repeat; gag, group-specific antigen gene; ppro/pol, protease/polymerase gene; env, envelope protein gene. (B) Schematic presentation of the subtypes of PERV and the recombination events and increase in the length of the LTR during passaging on human cells. Boxes in the LTR indicate sequence repeats. Joachim Denner, and Ralf R. Tönjes Clin. Microbiol. Rev. 2012; doi:10.1128/CMR.05011-11