Figure 1.

The IRES selection vector: schematic outline of the retroviral vector used in these studies. This vector expresses a dicistronic mRNA with EGFP as the first cistron and ECFP as the second cistron. This dicistronic mRNA is under the transcriptional control of the upstream long terminal repeat (LTR). Both upstream and downstream LTRs contain the U3 region from Rous sarcoma virus (lightly shaded boxes). The two libraries of random nucleotide sequences, where N represents A, C, G, or U, were inserted as indicated (see Materials and Methods). In the second library, N and N′ represent different random nucleotide sequences.