Episomal lentiviral DNA can act both as a substrate for Flp recombination and as a source of Flp recombinase for site-directed genomic integration. (A) Flp-mediated integration of plasmids into FRT-tagged HEK-derived cell line. The HEK/FGIP1 cell line was transfected with pLV/FRT-hygro and pCMV-Flp or a negative control and selected for hygromycin B resistance. (B) Flp-mediated integration of lentiviral DNA circles. HEK/FGIP1 cells were transfected with pCMV-Flp or a negative control one day prior to transduction with the IDLV/FRT-hygro vector. (C) Transient expression of Flp recombinase from integration-defective LV vectors is sufficient for site-directed insertion of donor plasmid. HEK/FGIP1 cells were transduced with IDLV/PGK-Flp or a negative control and on the following day transfected with pLV/FRT-hygro. (D-E) IDLV-encoded Flp catalyzes site-directed genomic integration of lentiviral DNA circles. HEK/FGIP1-6 cells were co-transduced with IDLV/FRT-hygro and IDLV/PGK-Flp or a negative control. Transfections/transductions were performed in triplicates (except for in panel E in which n = 1) and presented as mean value + SD.