We recently prepared to use SAM for screening experiments, but we found that the lentivirus titer of the package was always very low, and the cells were almost dead after adding puromycin. In addition, another CRISPRa library we used (Calabrese, from john doench lab) also had such a problem. Other lentiviruses we packaged have no problem, such as LentiCRISPR V2. We are very confused. Does anyone know the reason?
BTW, the cell we used is HepG2. In the preliminary experiment to measure the infection rate, we were not infected with dCas9-vp64 and MPH.