Fig. 1. Inverse PCR scheme. Integrated retrovirus is shown by a solid line with the 5' and 3' LTR regions represented by boxes at each end and the flanking genomic DNA shown by a dashed line. DNA is digested with SacI restriction enzyme to generate three products, but only those containing the LTR sequence are substrates for primer annealing and PCR amplification. The DNA is ligated under conditions that favour self-ligation and this allows PCR amplification using the forward and reverse primers shown by arrows in opposite directions on the LTR sequence. For each proviral integration two products will be formed, a constant product from amplification of the 3' viral sequence, and a variable product from amplification of genomic DNA flanking the 5' LTR. Retroviral sequence is shown by unbroken lines and flanking genomic DNA by dashed lines.