[Cell-biology] weird problem in transfecting 293 cells in 24-well plate

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I am doing transfection in 293 cells in 24-well plate. The transfection
reagent is Lipofectamine2000 (Invitrogen). The problem is after adding
DNA-Lipofectamine complex, in the some of the late wells (such as in Line D
and sometimes also in Line C) of the plate, the cells always die up to
10-30%. But in the wells of Line A and Line B, the cells always do not die
at all. The DNA problem is ruled out. I wonder what the reason is and how
to avoid the problem.