Transfection efficiency of DOTAP (A) or DC (B) DNA complexes formed at different ratios of lipid to protamine sulfate to DNA. One μg luciferase expression plasmid was mixed with the indicated amount of protamine sulfate prior to addition of the lipids. Complexes were added to 293A cells in the presence of 10% serum for 48 hours when the luciferase activity of the cell extracts was determined. RLU: relative light units. The mean and the standard deviation (error bars) of triplicates is shown.