Structure of the MSCV-FL retroviral vector. The FL cDNA is translated from retroviral LTR-directed 3.7-kb (full-length vector RNA) and 3.0-kb (spliced) transcripts that also contain neosequences. The 3.0-kb spliced FL mRNA is normally present as a minor species. The neo gene is transcribed from the murinepgk promoter as a 1.3-kb mRNA; p(A) indicates the polyadenylation site for all transcripts. Other abbreviations: SD, splice donor; SA, splice acceptor; ψ⁺, extended packaging region. Shown are the cleavage sites for the EcoRI (E) and Xho I (X) restriction endonucleases.