Efficiency of PEI-mediated gene transfer is dose-dependent. On the fifth day in vitro, sympathetic neurons were transfected with 0.2: 1, 0.5: 2.5, or 1: 5 μg DNA: μg PEI. Three days later, cells were fixed and stained with X-gal. Neurons expressing the plasmid encoding for β-galactosidase (LacZ+) were counted and expressed as a % of total cells scored (N=500). Data are expressed as the mean of three separate wells ± SEM.