Lentiviral delivery of protein tag-encoding sequences. The artificial exon sequence was incorporated into the U3 region of the lentiviral 3' LTR. During reverse transcription, a duplicated copy of the 3' LTR region replaces the 5' LTR region. Successful integration of this construct occurs into an intronic region of a cellular gene. Cre-mediated recombination involves two consecutive loxP-to-loxP recombination events and resulting in the excision of vector sequences between the loxP sites present in the 5' and 3' LTRs. This leaves behind the EGFP artificial exon as well as the R and U5 regions of viral 3' LTR integrated. ΔGAG, 5' portion of the gag coding region; RRE, Rev-response element; pSV40, simian virus early promoter; NEO, neomycin phosphotransferase coding region. Genomic DNA sequences are marked as straight black lines while vector sequences are marked orange.