The maximum size of sequence that can be accommodated between LTRs is 9.0kb. Taking into consideration the LV packaging sequence, RRE and cPPT required in the transgene expression vector, only 7.5kb of promoter-transgene sequences can be added to the vector. There is an inverse relationship between the size of the transgene vector and LV vector titer (Systematic Determination of the Packaging Limit of Lentiviral Vectors Mukesh Kumar, Brian Keller, Ndeye Makalou, Richard E. Sutton Human Gene Therapy. 2001, 12(15): 1893-1905).