In lentiviral gene delivery systems, transgene expression cassettes are commonly cloned without a polyadenylation signal to prevent disruption of full-length lentiviral genomes on mRNA maturation in producer cells.

Since lentivirus is an RNA virus, during the synthesis of the RNA genome to be packaged, if there is a polyadenylation (poly(A)) signal in the transgene, the RNA will be terminated prematurely.  Therefore poly(A) signal should not be added to any part of your insert because the poly(A) signal would lead to an incomplete transcript of the lentiviral genome.

Our validation data further confirms this, as we observed a significant drop in lentiviral titer when the BGH poly(A) signal was added after the stop codon of the transgene. Almost none of the infected cells expressed the desired phenotype, indicating that the poly(A) signal compromised the transcript’s integrity.