Schematic diagram of the shRNA-expressing vectors used for transgenic modification. (A) PB-EN3D2B, with a tandem array of cassettes that express the 3D-shRNA, EGFP, Neo-R, and the 2B-shRNA between the left and right transposon arms of the piggyBac transposon. Two loxP sites flank the reporter genes. (B) PB-ENLacZNH21, is identical to PB-EN3D2B except that the two FMDV-targeting shRNA cassettes have been substituted with non-specific shRNA cassettes. (C) PB-N3D2B, contains the fragment of two antiviral shRNA cassettes and the Neo-R reporter gene, which can be digested from the transposon vector for germ-line transgene; the transposon arms are not shown. The pairs of small black arrows indicate the primers used to detect the transgenic cell lines or mice. White bars indicate the PCR-amplified fragments used to determine the transgene copy number.