In vitro viral rescue assay in PBMC. Schematic diagrams of circle junction PCR utilizing primers outside the LTR, circle junction primer sets A and B (a) Primers are indicated by triangles. PBMCs were isolated at 50 weeks PI from both FIV-infected and uninfected control cats and cultured ex vivo for 10 days followed by PCR amplification for LTR circle junction sequences (CJ primer set B) (b). Expected PCR amplicons of 500 bp (single LTR, arrow) and faint 800 bp (double LTR, arrowhead) are present. Circle junction PCR products generated from cultured PBMCs from cats 184, 185 (FIV negative, negative control), 186, 187 and 165 are represented by lanes 1-5, respectively. PCR products were generated for 18s ribosomal RNA gene (d) from the same samples as in (c). All lanes have positive, appropriately sized amplicons (140 bp) for 18s rRNA.