Schematic diagram of the retroviral vectors LN, L-RRE-neo, L-TR/TAT-neo, and L-M10-SN. All vectors contain the 5′ LTR, the 3′ LTR, and the packaging sequence (ψ+) of the MoMuLV, and also the bacterial neomycin resistance (neo) gene. The vector LN, used as a control, only contains the neo gene. The L-RRE-neo vector has the RRE decoy sequences inserted 5′ of the neo gene; transcription is initiated from the 5′ LTR. The L-TR/TAT-neo vector has the ribozyme sequences cloned 5′ of the neo gene; transcripts are initiated from the 5′ LTR. In the vector L-M10-SN, the gene for the trans-dominant rev mutant M10 is inserted into the LXSN vector backbone, 5′ of the neo gene, and is under transcriptional control of the 5′ LTR, while the neomycin resistance sequences are under transcriptional control of the internal SV40 promoter.