FIG. 1.

Schematic drawing of the types of vectors used in this work. The transduced proviral forms are shown. All vectors shown carry an internal expression cassette for EGFP driven by the PGK promoter (PGKp). The following viral cis-acting sequences are labeled and shown as dark boxes if derived from HIV-1 or as crosshatched boxes if derived from MLV: the LTRs with the constituent U3, R, and U5 regions, the leader sequence containing the major splice donor site (SD) and the packaging signal (Ψ), the portion of the gag gene included for optimal packaging efficiency—crossed out to indicate that the reading frame is closed by mutations—the env-derived splice acceptor sites (SA), and the RRE for the HIV vectors. The deletion of enhancer (Δenh) or enhancer and promoter sequences (Δenh/pro) from the U3 region of the LTR is indicated for the MLV and the self-inactivating ΔU3 HIV vectors, respectively. The types of vector transcripts discussed in the text are indicated by dotted arrows.