We have made a lot of pLKO.1 based lentivirus with no problem. I am currently trying to make the pLKO-tet-on based lentivirus using pSPAX and pMD2 vectors for packaging in HEK293TLA cells and we seen to be getting no virus when I try and titer the virus in HEK 293 TLA cells selecting with 1ug/mL puro. Are there any tricks to playing with virus made with pLKO-Tet-on plasmid vs the pLKO.1 plasmid.