FIG. 4.

Schematic representation of the bicistronic retroviral vectors. The VL30m-MLV-based retroviral vectors are built on a pBR322 backbone. VL30m corresponds to the 5′ RNA region of the mouse VL30 retrotransposon, VL30r corresponds to the 5′ untranslated region of HaMSV (8), and MLV E+ corresponds to the extended packaging region of MLV (5). Placental alkaline phosphatase (plap) and neomycin phosphotransferase (neo) were used as marker genes. The control vectors pEMCV-CBT4 and pRev-HW3 possess two IRESes (53, 84), the first from MLV (7, 87), which also directs packaging (E+), and the second from EMCV or REV-A (53). In all cases, numbering is with respect to the genomic RNA cap site (position +1).