Design of retroviral constructs expressing Myc and Bcl-2. (A) Murine c-Myc was inserted into the MSCV-IRES-GFP backbone, which uses the long terminal repeat (LTR) sequence to drive high expression. Dark bars represent Myc Box I and Box II. Striped bar represents the basic helix-loop-helix leucine zipper motif. Arrows represent LTRs. Psi represents the viral packaging sequence. B indicates BamHI; E, EcoRI; n, NcoI; H, HindIII; X, Xba. The murine Bcl-2 coding region was inserted into the viral backbone in a similar fashion. A construct coexpressing both Myc and Bcl-2 was generated by replacing the GFP sequence in the Myc construct with the Bcl-2 coding region. MIG indicates MSCV-IRES-GFP; MIB; MSCV-Myc⁺Bcl 2 (B) Expression of Myc and Bcl-2 in transfected 3T3 cells. Cells in exponential growth phase that were transfected with either GFP or Bcl-2 express similar levels of c-Myc. The cells transfected with either Myc or Myc/Bcl-2 construct express significantly higher levels of c-Myc. Only the cells transfected with either Bcl-2 or c-Myc/Bcl-2 express Bcl-2 protein. (C) IL-3-independent growth of BaF3 cells. Only cells transfected with both Myc and Bcl-2 (black line), but not Myc or Bcl-2 alone (gray line and long dashed line), grow in the absence of IL-3. Cells transfected with GFP alone do not grow in the absence of IL-3 (dotted line).