I am trying to generate a stable cell line using lentiCRISPRv2 infection with my desired gRNA. This plasmid has the Puromycin selection marker. I used 2ug/mL Puromycin for selecting cells, but some cells in control group (No viral infection) are still present even after 3days of Puromycin containing media. I changed the media every 24 hrs. Can anyone guide in this how these cells are becoming resistant?