Schematic diagram of plasmids used in this study. (A) pCS-kRI, used for polycistronic expression of phiC31 RDF and integrase (Int) in mammalian cells. (B) pCS-kI, for Int-only expression. (C) LR-OFF, used to detect attL-attR recombination (LxR). (D) PB-OFF, for detection of attP-attB activity. (E) PB-ON, a positive-control vector used for normalization of recombination activity. (F) pN2B-RTL-puro, for construction of cell lines that permit assessment of genomic LxR activity. Unless otherwise indicated, all cloned att-sites are 50 bases in length.