Before you choose the packaging vector mix for generation of lentiviruses, you would like to know how the lentiviral genome is expressed. There are at least two ways that the lentiviral genome is transcribed: (1) lentiviral genomic RNA is transcribed from a chimeric 5′ LTR in which the HIV-1 promoter is replaced with RSV or CMV promoter; (2) lentiviral genomic RNA is generated from a wild-type 5′ LTR. Please note that the wildtype 5′ LTR requires the gene of Trans-Activator of Transcription (Tat) to be present in the packaging vector in order to be highly active.