Template Switching (TS) rate of foamy viral replication. (A) MD9-derived PFV vector viruses (KG81 and KG82) expressing hygromycin and neomycin resistance genes under control of a SFFV U3 promotor. CASI/II are cis-acting sequences required for FV vector transfer . KG81 carries a point mutation in the hygromycin resistance gene that abolishes its function and destroys a SacII restriction site. KG812 carries a point mutation in the neomycin resistance gene that abolishes its function and destroys an EheII restriction site. The two mutations are 1 kb apart. (B) Distinction of TS events from superinfection with KG81 and KG82 by restriction pattern: amplification of the proviral sequences by PCR generates a 2.2 kb fragment with a SacI site at position 500 and an EheII site at position 1550. Amplicons of TS events carry the two intact restriction sites and show a restriction pattern depicted in the upper box, whereas amplicons of superinfected cells show the restriction pattern depicted in the lower box. (C) Representative digests of 10 clones from the TS experiment. All 10 clones show the expected pattern for TS events. Upper lane 3 and lower lane 2 show incomplete digests.