Luciferase expression in transfected cells in culture. (A&C) HEK 293 and (B&D) mouse ES cells transfected using either lipofectamine 2000 (A&B) or the Helios Gene Gun (C&D). The top row in all sets of plates shown did not receive the luciferin substrate whereas it was provided to all wells in the bottom row of each plate shown. The leftmost column of each plate contained mock-transfected cells. The middle column was transfected with pNcx1-LUC, and the rightmost column of each plate was transfected with pCMV-LUC.