Transfection efficiency depends on the DNA: PEI ratio. Rat sympathetic neurons were cultured in 12-well plates and transfected as described in "Methods." Various ratios of μg DNA to μg PEI (0.2, 0.4, 0.6) were used, as well as DNA alone (2 μg) and PEI alone (5 μg). Three days post-transfection, cells were fixed and stained with X-gal. The number of cells expressing the plasmid encoding for β-galactosidase (LacZ+) was counted and expressed as a % of total cells scored (N>500). Data are expressed as the mean of three separate wells ± SEM.